Would you welcome witty retcon?

It could be done (or could have been done) in numerous fashions to evolve a plethora of character or story arc interpretations.

As for me, I have three particular scenes in mind.

Sure. The Vader coronation scene in Ep. III, Yoda's decision to run from his duel with Palpatine in Ep. III and Palpatine's death rattle in Ep. VI. All of them have ostensibly simple explanations that viewers have been led to accept because some of them were explained in what are now Legends novels and some are just Occam's Razor kind of conclusions; logical and without obvious reasons to pursue alternative theories.

In Vader's coronation, we hear Palpatine's voice alter into something demonic. He speaks a few lines in this altered voice and then Anakin says something like, "I will do whatever you ask. Just help me save Padme's life. I can't live without her." Palpatine's response, which is the third and final time he discusses Plagueis, replies, "To cheat death is a power only one has achieved. But if we work together, I know we can discover the secret." Except when he says that line, his voice alters yet again. Conventionally, the explanation is that there is nothing to this second alteration, that it is the power of the dark side roiling inside him, causing his voice to continue to evolve.

In the Yoda/Palpatine duel, the eye test tells us that the duel is a flat draw. Yet Yoda retreats. Conventional logic is that Yoda has an epiphany that Palpatine - who is his Force equal - is now no longer his equal because he'll be commanding the Republic's army/navy and that he's branded the Jedi outlaws and so Yoda in this very quick moment adds this all up in his head and decides to live to fight another day.

In Palpatine's death scene, we witness a strange, blue energy rushing up the shaft and then back down the shaft. There was at one time an explicit answer to what this was. When Sith Lords die, their Force energy bursts forth out of their dead bodies like a gasoline explosion. And to a world of Chaos they go. This explanation, however, is no longer Canon and there has been no Canon explanation offered since.

All three of these things are seemingly insignificant to the main story arcs. Sure, Yoda leaving is a moderately important plot point, but the scene in question does not overtly offer an explanation, so determining something different about its meaning does not invalidate anything important about our original interpretation of the scene. Retconning or perhaps revealing something that was intentionally easter-egged only adds to the fun we originally felt when watching the movies.

There are many cool things that Snoke = Plagueis brings to the table: Luke gets to confront his grandfather. We come full circle to meet the architect of everything that happened in the first six episodes. We bring the idea of The Sithari to the films. We allow for the opportunity to more properly explain what Kenobi meant when he said "I shall become more powerful than you can possibly imagine," giving more layers to what it means to be a Force ghost than just mentoring. And it provides a very interesting subplot that we never knew existed: The Jedi and the Sith (unbeknownst to each other) were both racing to unlock the key to immortality at the same time; Qui Gon in his way and Plagueis in his way. What is really neat about this idea is that, according to the Plagueis novel, Maul slays Qui Gon and Palpatine "murders" Plagueis at exactly the same time! Qui Gon was blessed by the Force at this moment to counteract Plagueis' unnatural corruption of the cycle of life. Balance was the way forward yet again.

But other, less important possibilities arise with a Snoke = Plagueis revelation and examples of those include the three scenes I mentioned above and the idea of easter eggs or retconning. According to Legends material (you have to read several books and piece together common threads from each to get the full picture...it's like reading the Bible and understanding that text in Jude has an impact on the meaning of text in Genesis), Plagueis transferred his spirit into Palpatine. If this is revealed to be the case in the ST, then perhaps the second alteration in Palpatine's voice during Vader's coronation is explained by the fact that Plagueis' voice is coming forward. When I listen to the audio, I hear two distinct voices. And the second, newer voice, sounds just like Snoke. And instead of Yoda retreating because he decides instantaneously during the heat of the toughest battle he's ever fought that the Jedi have become public pariahs and its in his best interest to flee (this is a lazy interpretation of that scene in my opinion), instead, we find out that when Yoda pushes Palpatine back with the Force and Palpatine gets that look on his face, like, "Whoa, I better tap into my full power now," Palpatine's mental block that he (and Plagueis) had been using to hide themselves and their plot from the Jedi was weakened due to lack of concentration, and Yoda was (finally) able to see things for how they truly were, which is to say, he was in fact over-powered by the spirit of Plagueis and knew that, while he could go toe-to-toe with Palpatine, he was outmatched by what lay beneath the surface. And finally, when that blue energy comes shooting up out of the reactor shaft, what we're actually seeing is Plagueis' spirit escaping Palpatine's body and searching for a new host. The blow-back - when the blue energy recedes back down into the shaft again - is Palpatine's spirit trying to tag along with Plagueis' and Plagueis pushing him back to the abyss of his own body.

Oh, also, Snoke = Plagueis provides a perfect rationale for Luke disappearing after Kylo Ren's betrayal. It's the same reason Yoda ran in Ep. III. You and I both know that Disney will never try to sell us on the idea that Luke quit because he's a wimp or because he was so psychologically drained from Kylo's betrayal that he couldn't hack it anymore. Uh uh. No way. Luke has already proven time and again that he's willing to march into certain death to help his friends. He would never abandon them unless there was no other choice. Plagueis gives the viable option for retreat. How can you defeat the immortal? Even Yoda couldn't do that. The only way to solve a problem like that is to find the origin of the Jedi in order to find some as-of-yet-unknown power or method to defeat such an enemy.

So anyway, those are my examples. As mentioned before, there are dozens of other opportunities to use this tactic. Those are just the ones I see as the greatest potential to have a major impact on the PT and OT without tearing down any of their established tenants, but yet, at the same time, gives us reason to go back and watch the first six movies with a different perspective.

Midi-chlorians never bothered me.  It was the scene where Qui Gon explains what they were to Anakin that gives midi-chlorians their bad reputation.  The way that scene was shot, acted and lit, it stood out from other scenes in the movie.  The dialogue and pacing sounds and feels like a cheaply-made infomercial.  The way it pops out of the movie, it also reminds me of the retro scene in Dodgeball where Patches is teaching dodgeball to the little kid.  Except that was supposed to look that way because it was supposed to be campy and funny.  In TPM, the explanation scene is unbearable.

But honestly, I almost kind of like the idea of midi-chlorians introduced as an idea in SW.  Necessary?  No. Indeed, we all accepted that the Force was quasi-supernatural (or at least meta-physical) and we would have been content to keep on believing that.  I'm the type that actually prefers a physical link and way of quantifying what are otherwise "supernatural" abilities.  So the midi-chlorians were a positive supplement to my SW understanding.  I also think they make more sense than what we as fans had all kind of glossed over.  No matter what, there must be a bridge between the physical and spiritual when physical people are tapping into spiritual powers.  There must be.  This is only logical.  I guess most fans just didn't want to hear the nitty gritty detail of how Jedi can actually tap into the force.  It's not a miracle.  There is a physical explanation and I'm in the minority for appreciating that explanation.

The Force remains spiritual, so nothing changed there when midis were added to the lore.  One's control of the Force is still based in belief/will power/discipline/training.  All of these things impact how well one can use the Force in addition to how many midis one has.  The combination of those things add up to how well one can use the Force.  You seem to be suggesting that the midis and the Force are one and the same.  They are not.  Midis are bridges between the physical body and the spiritual Force (although, Plagueis seemed to think they were interlocutors instead of helpful bridges).  They are what allows your physical body to go through the motions of things that are not bound by physical constraints, e.g., spiritual power.

"Most importantly, if using the force depends on your midichlorian count then no matter how much will or belief you have you will never be able to use the force if your count is very low."

Well, yes, of course.  This why most people in the SW galaxy are not Force sensitive.  Not everybody has them.

Why wouldn't all the Jedi just takes blood infusions of high midichlorian infused blood? For that matter why not have everyone/anyone do it? Every one could be a force user with a blood transfusion.

Because midis are bound to the spirit and body.  It is a multi-pronged unison at work when one uses the Force.  And while bodies and blood can and have been switched in SW lore, spirits are unique and cannot be copied.  When midis are transferred to a non-Force-sensitive being, they die.  Plagueis tried this, but of course, that book is Legends now.

In conclusion, nothing about the Force changed from the OT to the PT.  It was just explained more scientifically.

Sure, it was unnecessary.  "Is it necessary for me to drink my own urine?  No, but I do it anyway because it's sterile and I like the taste."

"Middleman" is sort of accurate.  The body can only operate in the physical.  The Force is spiritual.  Midis can communicate/operate with both sides, so they allow physical beings - who would otherwise be unable to do anything beyond the physical - to touch/manipulate the spiritual realm.

Training is totally necessary.  Consider TPM's Anakin.  Higher midi count than Yoda.  Yoda would have killed him in a duel in less than two seconds.  Yoda would have continued to whip Anakin until he was fully trained by Palpatine.  Kenobi beat Anakin on Mustafar.  Training and discipline still matter very much when it comes to application of the Force.  Or, an even better example of this is Leia.  Leia undoubtedly has an extremely high midi count due to her lineage and yet the only Force power she's exhibited is telepathy and she would get whipped in a duel with a low-level dark sider like Asaj Ventress.  No, your body's level of midis only puts a cap on how powerful you can be.  A person with mid-level midis and high-level training/discipline/self-control will whip a person with high-level midis and no training.

Midis are just the wire connecting one end of the phone to the other side so that you can talk to the Force.