People need to temper expectations on this a little , there's really no reason to expect the final roster of this game to be any bigger day 1 than Marvel 3 was on day 1. It's perhaps not getting the exact same treatment as SFV (because great though the game may be , it's not been as much of a successful experiment as Capcom was hoping) but , they might have DLC packs that just pop up every few months that contain another 2-4 characters in a shot and they'll keep releasing them as long as the game stays popular.
So basically, I'm sticking with my 32 characters guess - 16 from Marvel and 16 from capcom with a brand new final boss character to replace Galactus / regular Thanos /whatever that thing in marvel 2 was.
The trailers have already established that the initial roster isn't against re-purposing x-men characters into "inhumans/meta-humans" like Captain Marvel (who is basically phoenix).
4 characters from SF makes sense , 2 from darkstalkers tops , Megaman X , 1-2 from resident evil, Strider ( hasn't missed one yet, why start now). That already makes 10. Obviously Devil May cry can still supply Dante so you still have room to toss in 5 more characters. Of these we have 3 confirmed of course - Ryu, X , Morrigan. I don't believe the leak so I'm guessing we'll have Chun Li as the other "good" with Bison and Akuma as the "bad SF characters. X may well get Sigma (although that other megaman slot tends to go to tron bonne) , I'd love for Dimitri to return opposite Morrigan (and give him his midnight bliss special). For Resident Evil ,change it up and throw in Claire Redfield but keep Wesker. Strider can go solo , DMC could toss in Dante and Vergil. That still leaves 3 slots they can fill with arthur , pheonix wright, a monster hunter thing, breath of fire character, etc.
Throwing in every main character from an MCU film also adds up to quite a bit - Iron Man, Captain America, Thor, Hulk, Hawkeye, Black Widow, Black Panther , Scarlett Witch, Vision, Ant-Man, Spider man, War Machine, Dr Strange, Captain Marvel, Wasp. Not to mention the Gaurdians - that rumored Rocket/Groot 2 for 1 character actually sounds totally plausible. Winter Soldier, Whiplash(could play like omega red), Dormamu, Ultron, Loki, Thanos, Red Skull. Boom - that's 24+ characters right there. That's just drawing from the MCU.
Of course, that probably BS rumored 60 strong cast could allude to 1 thing - Tekken style clones. I mean, Black Panther can just play identical to Wolverine but if they add X-men stuff back in ? yeah it'd just be a skin and voice swap I think, otherwise the same character.
Gotta admit though, I'm more curious just who they throw in from Capcom at this point because the Marvel side seems so obvious already.