I mean, carryovers from that saga have done their duty. Personally, this is like G1 Transformers post-animated movie. Which because both cosmic, isn't far off.
Bring me X-Men & Fantastic Four. New blood to ride with.
I think they are phasing out the original big three: Ironman, Thor, and Captain America. Tony and Steve’s narratives have wrapped up and Thor is being demoted to a mere mortal with Jane taking the Hammer. Hulk really hasn’t had much to do in the movies other than smash. David Banner himself is a side character used to support to other three. The others like Black Widow and Hawkeye have never been the central focus, so they’re stories are fresh. And the GoTG have their own narrative outside of the Avengers.
However, I do think the Fantastic Four will show up soon. Maybe initially in the TV shows. Out of all the other Marvel Hero groups, they’re my favorite. I’d like to see a good reboot for them.
An Avengers vs X-Men would make for a great one-off movie, though they’re characters deserve their own narratives separate from the MCU folks.
I don’t know if Thor is becoming mortal. There are plenty of Asgardians that don’t have weapons bestowed upon them that are still beyond mortal. Unless you’re referring to something confirmed I haven’t heard about.
Rocket is the one that's the safest imo, he's actually had storylines through all the films including the Avengers movies, he's also one of the most beloved of the characters and most marketable, plus he is a package deal with Groot, on top of that Bradley Cooper & Vin Diesel are probably cheaper than the other cast as they are just voice over work, so they aren't doing 2 months worth of shooting but like 2-3 days in a sound booth, but they are also big names to add onto the poster, so yeah I figure Rocket & Groot are safe, but then Groot "dying" again to bring back Baby Groot means more Baby Groot merch and people love Baby Groot, so who knows, but that would be a total cop out imo to kill Groot again.
IF Gunn is meaning a MAIN CHARACTER is going to die and it is the end of the GOTG series, i could totally see Rocket being killed as a "subvert your expectations" style twist.
If it was just up to Gunn then maybe, but I don't see Disney chancing it myself, not after so many things have had that blow up in their faces in recent years, including SW, plus they still want to pump out the merch, as well as have the characters viable for crossovers, I don't see Rocket as the smart choice to do this, you think they have Thor set up to be a part of them as the Asguardians Of The Galaxy, that means they have a replacement male lead or comedy muscle they can slot him into, that to me makes Drax and Quill the more obvious ones to kill off, if he does mean a main character like you said, but who knows.