Max has been live for a little bit now, showcasing all of the changes...Art, characters, music, extra levels, etc.
He's also going to be going over gameplay changes as well. I will edit this post going forward.
Edit:
-Steam deck compatibility confirmed. Some of the team was playing the latest version on it already.
-New menu art, new stage art, new character selection screen and art
-More clarity of moves due to art changes
-New end of battle screens, new beginning of battle screens, new UI art, new KO effects
-New super transitions
-New announcer voice
-New Venom voiceover
-Character models now have a brighter style, hand drawn linework for each, including remade models (Chris's new model has his RE 5 face)
-Brand new alternate costumes (including obscure references like Vergil MM Zero)
-Costume Pack DLC must be purchased to access the above. They are not allowed to make new outfit models outside of what's available.
-Max asked the director of Strider 2014 if it was okay to use that specific strider model/costume and he okay'd it. They had to rip the model from the 2014 edition and make it fit within the new artstyle. It works including the electric scarf. Also costumes that reference Strider Hien and Hayato from Plasma Sword.
-Spencer has his old school Nathan 'Rad' Spencer outfit, the P2 outfit, and a special "Nuke'em" outfit.
-Captain America was a huge change in model, went more for 90s comic-style Cap.
-Doctor Strange is also a redesign, more 90s comic-style in design
-Rocket Raccoon was also made to look more cartoon/comic-book esque rather than the 'realistic raccoon' look he had originally.
-Gave Iron Man a War Machine alt costume
-New music tracks
-Custom menu themes
-Dante has his DMC 3 combat theme now (yes, the one everyone likes)
-Mega Man X's track is really nice, heard it on Saltybet before not sure which game it's from
-Unsure of where the Zero theme comes from (I really need to play the X series beyond part 3) but it sounds nice
-Cap's theme sounds like it's from the old arcade game I think
-Monster Hunter has the traditional MH theme track now
-Hawkeye's theme might be from the Sega genesis game (some of these tracks are deep cuts)
-New balancing
-Games moves a bit faster, in between MvC:I original and MvC3 speed.
-There will be more balance patches in the future.
-Dramatic adjustments to stone system
-Dramatic adjustments to tag balance
I did not list every change, and this is more of a phase 1 launch. They will still be doing more after launch, and he will fund whatever the team wants to do post-launch.