I miss Dad & Dan.
Um... there was a Marvel (TV) movie in the 90s. It had David Hasselhoff as Nick Fury. It was not well received, mostly because Hasselhoff did not have the proper gravitas for the character.
I'm not liking John Stamos as Bucky. This will knock the socks off some people here, but have 90s Robert Downey Jr as Bucky over John Stamos. (BTW, IIRC, Winter Soldier wasn't around in the 90s)
And as someone else pointed out, 90s Keanu Reeves wouldn't fit as Doctor Strange. Liam's a better choice.
Joe Pesci could work as Rocket, but I'd rather see him play Hammerhead. Marisa Tomei as Nebula over Demi Moore.
You speak of gravitas, but I have difficulty seeing Tomei as a brutally mean Nebula. Demi "GI Jane" Moore could pull it off... even 90s Nebula.
Maybe Jeanette Goldstein (Aliens' Vasquez), but that's kind of a typecast.