Both fill different needs. With characters that have 50-60 or more years of history, variants let you cover them at different points of their life, which is then compounded by the Marvel multiverse having different versions of the character as well (most notably for MPQ, with the MCU versions of characters).
For some, for example, I’m glad we got a 5* Storm to let her exist as a true “Omega Level” mutant at the top tier of play. Same with Iceman (though I would argue this is our first true 616 Iceman, since the time-displaced Teen Iceman isn’t the same)
There are other characters that could be elevated to that tier, Magneto and Scarlet Witch as obvious options, but I’d also like to see an MCU Scott Lang (let him go full on Giant Man while we’re at it), and would not oppose another Wolverine (a 616 version, not Old Man Logan). And it doesn’t strictly have to be one-way like that, either. I know we’re not likely to get more 3-star characters, but that’d be a good spot for, say, a Marvel Girl Jean Grey, or to make long-time 5’s like Green Goblin or Silver Surfer accessible to newer players by giving them a lower-tier version.
Oh, and I always thought we should have added an Unworthy Thor while Jane wielded the hammer in the comics, to let them fight together (now I’d rather see an MCU Endgame Thor, personally).
OH, and going through both Infinity War and Endgame without an MCU Iron Man feels like a missed opportunity, both of those suits could do some cool stuff. And we’re close to having a good reason for another comic variant of Iron Man, too.
But there are plenty of spots for new characters.
Some to cover missing iconic characters. Red Skull should at least be in as a boss, but having him playable would be fun as well.
I would really like to see Arcade, solely because his shtick seems like a good excuse to try weird game mechanics for MPQ.
I’m always down for more web-tile users. It feels like an under-utilized game mechanic (I still wish Eddie Brock’s green counted web tiles as enemy tiles, to give him a unique boost against Spider-people). And then you get to start to blur the lines between new characters and variants. Ben Reilley, Silk, Mayday Parker, Anya Corazon, Miguel O’Harra, etc.
Beyond that, I’d like to see Forge, Franklin Richards, a couple Spidey villains (Rhino, Scorpion, Shocker, Electro, etc), and ANY version of Hank Pym. Feels weird not having him represented at all. But the list could go on and on and on (Elixer, Goldballs, Mr Sinister, Marrow, US Agent, Viv Vision, Hulkling, Quasar, any of the other six Captain Marvels, Blue Marvel, Mandarin, Nightmare, Dormammu, Toxin, Anti-Venom, Lizard, Richard Rider, Adam Warlock, Mephisto, Alpha Flight… you get the point)