Prefacing this: Both side's MvCI rosters include DLC. I actually have no idea who is launch because it's still up in the air sans no X-Men. My MvCI info is around 3 months old btw. Source said wait til New Year's to leak. I wanted to do this on Xmas. With that said...
MARVEL: Iron Man, Captain America, Captain Marvel, Ms Marvel, Thor, Loki, Hulk, Hawkeye, Black Panther, Black Widow, Ant-Man, Spider-Man, Venom, Daredevil, Iron Fist, Luke Cage, Ghost Rider, Punisher, Ultron, Thanos, Star-Lord, Rocket (w/Groot), Doctor Strange, Gwenpool, Squirrel Girl, Fing Fang Foom (Giant Character), Wolverine, Magneto, Storm, and Doctor Doom.
CAPCOM: Ryu, Rashid, Chun-Li, Zangief, Gill, Morrigan, Talbain, Tessa, Mega Man X, Sigma, Roll, Strider, Jin, Captain Commando, Edward Falcon, Asura, Gene, Dante, Leon, Ada, Wesker, Nina (BoF2), Spencer, Simon Blackquill, Phoenix Wright, Rathalos (Giant Character), Masamune Date, Batsu, June, and Frank West.
Cinematic trailer is indeed Ultron infected with the Sigma Virus and the main villain of MvCI.
Wolverine, Magneto, Storm, and Doom are all DLC and not making the base roster.
Some Street Fighter characters might be held off on MvCI as DLC to make an X-Men vs Street Fighter DLC themed pack.
MvCI's director Norio Hirose wanted the Capcom cast from MvC to return since he programmed that. Hence why they're part of MvCI (except Mega Man is Mega Man X instead).
Ono suggested Ingrid and R. Mika for MvCI.
Combofiend suggested and got Spencer on MvCI in at his own request, as the team obliged.
Phoenix Wright was almost nixed on in favor of only Simon. The team felt he was a failed idea on UMvC3, but wanted to redeem him. They choose Simon because he's more straightforward and relevant (AA5 & 6). Franziska hasn't been on any lately, so no priority.
More than one BoF character was considered for MvCI. Bleu and Fou-Lu namely. Bleu almost got in with Nina. Katt, Rei, and Garr were all considered as well. Ryu never was. For fans of it, if you want more Breath of Fire characters later on MvCI, make noise. They'll listen.
Dante isn't based on DmC (reboot) version, but classic Dante for those who are worried. Dante isn't based on the reboot version because DMC5 will be announced in 2017.
Batsu almost got passed over in favor of Akira and Yurika as the Rival Schools rep.
Edward Falcon was included on because Power Stone fit the Infinity Stone motif AND Power Stone is coming back.
Leon was included to hype up the RE2 REmake.
They went with Tessa over a 3rd Darkstalkers character because of her storyline connection with Doctor Strange (which was previously teased on UMvC3) that they want to expand upon in a meaningful way. Both are scientists who dabble in magic with an interest in the unknown.
Masamune Date got in to appease the Japanese fanbase and garner interest from them.
When it comes to Zero's exclusion, as far as I know, Zero's getting the Mega Man treatment this time around. Which is an odd exclusion given X is there. And back to 2vs2. But Zero got in TvC and both versions of MvC3 without X. So maybe that's their view? I can only assume they thought 3 MMX characters were too many. Zero might show up in the story mode though. I haven't heard anything for or against it, actually. But hold hope for that and DLC maybe. It'd honestly be surprising if Zero didn't even show up in the story. But X is the one of the focal storyline characters on MvCI, so they could be looking at it like not wanting to take away from X's glory. X might've been chosen and highlighted in such a way in spite of Zero. I'm really not sure. But Zero's exclusion, to me, seems to be the head scratching disappointment like Mega Man's was on both MvC3s.
Howard the Duck almost got in in place of Squirrel Girl. He just barely missed the roster, but fan demand could easily get him on MvCI.
As said, Rocket now has Groot included as an assist like Maya does for Wright in his moveset on MvCI.
Ant-Man is no longer in Hawkeye's MvCI Hyper. Hawkeye has a brand new one entirely. Wasp is in Ant-Man's moveset btw.
There will be characters not in MvCI's launch roster nor current DLC plans in the story mode ala SFV.
Akuma isn't part of the roster or even planned at the moment as far as I know. Could change though. I'd assume Akuma could make it in because of possible XvsSF pack plans.
Fans will likely wind up dictating more X-Men DLC than the 3 they have planned. They are aware of that already. I wouldn't be surprised if they launch with only Ryu and Rashid. And then make Chun, Gief and Gill DLC in that XvsSF pack since 3 of each that way.
Lastly, my name is actually a hint as to when it comes out... At least that's the aim internally, barring any issues that arise.