I always thought we had a bunch of missed opportunities for characters for Infinity War. Widow and Spidey were solid, Okoye was right after but also linked to some BP stuff going on (Blu-Ray release at least, maybe an award show or at least nomination, I forget). We got Wong and the Black Order via Supports.
But a Stormbreaker Thor, nanotech Iron Man, maybe a Doctor Strange showing off his cool new tricks we see on Titan. They all would have worked as 4s, since none have a primary version at that tier (Thor’s is Jane and not the Odinson himself, and Iron Man is Hulkbuster, not a standard Iron Man suit). Same for Cap, I would have put the Infinity War cap as a 4 as well (and I would have downplayed those goofy digging mitts, I know they’re supposed to be a “Wakandan shield” to stand in for his normal one, but they’re goofy and I hate them). And I would have added a 4-star Thanos back then, with an armor-less MCU version. I would have ignored the 5/4/4 pattern or the idea that 5’s are how the game makes money, and gone to town with 4-star characters.
Now that we’ve seen Endgame, some of these could still work. We don’t have an Infinity War Thanos (who I prefer over the Endgame Thanos, personally), and there’s still room for Doctor Strange or Iron Man to have newer and more up-to-date MCU versions.
Fat Thor is probably best accomplished with a costume. Give a costume to the 3* character (which people already call Lazy Thor or sometimes Fat Thor, long before the MCU version). And the Stormbreaker support works with him, so that’s cool.
The “big glaring omissions” are a new Iron Man, because the last two Avengers films provide all sorts of cool toys for him. The super thruster, the sword and shield, the extra wing things that can fly off and add to repulsor blasts, the nanotech in general, both the Infinity War and Endgame designs.
And then Worthy Captain America. Or "Captain America (Worthy). Obviously. Don’t even care if that means a third 5-star Cap. We’ll call it the second by pretending that the goofy digger mitts don’t exist, and everything will be okay.
And THEN there are a couple others Endgame could add in. Seeing a solo Rocket in his new outfit would be cool, but what’s more likely is a Rocket & Teen Groot costume, I think. Scott Lang could use a 5* version with Giant-Man powers. Hope could use a 5* that isn’t complete and utter garbage. Rescue and War Machine would both be rad. Maybe a “good Nebula” costume (for that matter, a whole mess of time-travel suits could be good costumes, assuming the devs ever figure out how cosmetic items are supposed to work in games and stop pretending they’re worth most of a hundred dollars…)
If I absolutely had to give a short list, it would be Worthy Captain America, a new Iron Man from either film, and reasonable access to the Infinity Stones. But you could go on and on and on and on just dabbling with the main characters before you even get into other things missing from an MCU standpoint (as MackDan just listed).