Spencer isn't a fan favorite anymore? Morrigan became one of the biggest characters in UMvC3. And fans were clamoring for the likes of X & Sigma in UMvC3. Frank West also had plenty of hype & people who loved his playstyle in the MvC3 series. And this time, we got Strider in the launch lineup.
Like, honestly, the biggest omissions on the Capcom side is probably Wesker & Vergil, and they just so happen to be some of the most played characters in the Capcom side in Marvel 3.
Also, several characters, specifically Chun-LI, looks infinitely more viable in Marvel Infinite than she did in Marvel 3. They totally got a feel for what people wanted - but they had criteria for who they'd pick & why.
Let's be honest, we know Spencer got in specifically because Peter is on the MVCI team. I doubt it has anything to do with being a fan favorite. Morrigan is a given since she's in every crossover. X is in, yes. Strider, yes. Sigma is DLC and should've been in day 1 considering he's a fellow antagonist of Ultron. Frank West is another given. I like Arthur and Firebrand, but they have no business both being in a 28 character roster.
No Wesker, Vergil, Nero, Phoenix Wright, Zero, Akuma, Haggar, Jill (MVC2), Viewtiful Joe, Ammy, Gene, Asura, Rival Schools rep, Power Stone, Sengoku Basara. This is what crossovers are supposed to be about. Bringing in various franchises to rep the Capcom side. There's none of that. This is infinitely more scarce than anything in MVC3 and it sucks.
I also think it's way too early to talk about viability. Chun-Li looked viable when everyone was running her for her Lightning Legs assist in beta MVC3. What happened by release? Dropped like a bad habit by nearly everyone.