In your opinions the Tatsunoko vs. Capcom roster may be fine, but I am looking at this from the "big picture" perspective. TVC is a fairly niche title, and MVC3, if and when they make it, is gonna be huge. If the game was going to have a giant roster, then they certainly could throw in whatever characters they want, but if they can only have about 15 characters from each side (which I am very confident will be the case), they can't afford to suck up all the slots with schmos that most gamers either have never heard of or don't give a crap about. Let's have a look at TVC's Capcom roster, shall we?
Ryu: Just about every gamer knows him, so he's a lock
Chun Li: same as above
Batsu Ichimonji: I understand that Rival Schools is a cult classic, but I've never met anyone who plays it regularly, and i had to look this guy up to even know who he was. Pass.
Alex: I know who he is, but only because I played Street Fighter 3. Most people haven't though, and personally I'd rather keep a slot open for Guile. Pass.
Morrigan Aensland: She's been in pretty much every Capcom crossover game, so she's pretty much a lock.
Sōki: The last good Onimusha game was Onimusha 3, which makes this character irrelevant to 90% of gamers. Pass.
Megaman Volnutt: I'd rather have the classic Megaman (or Megaman X), but I'd settle for Volnutt. So I'll say "tentative lock."
Roll: Seriously, who wants to play as Roll? She's not even a fighter. Pass.
Saki Omokane: Um... Does this even need an explanation? No-one knows who this character is. Pass.
Viewtiful Joe: An awesome character from a modern Capcom classic. And he's a superhero! Lock.
Yami: Given that Okami is a huge cult favorite, I guess it's fair for it to get some representation in MVC3. 50/50
So that's approximately 5.5, half of the 11 character Capcom roster, so that would be either 5 or 6 characters that MVC3 would likely have in common with TVC, leaving up to 10 more slots open. Personally, I'd rather see the rest of those 9-10 slots go to characters who are actually worth giving a crap about, rather than wasting them with obscure characters who have virtually no recognition outside of Japan. If they ported over the entire cast of TVC, realistically there'd only be 4 slots left for characters that are actually popular, and that just plain isn't good enough. Use logic, people!