Tatsunoko vs. Capcom/WMG

Everybody's aware they're being watched on some sort of television program, but decide to make the best of it.
They know that people are watching them, day in and day out, so they decide to try and tell people that everything they do in their lives is not fight, but then some jerk who wants publicity will pick a fight.

To follow from the above theory, the TV producers stop caring round about the Tatsunoko saga.
I mean, what was the point of the characters fighting Yami if its never gonna be acknowledged again?!

Yami escaped to the Tatsunoko Vs Capcom world to get back to full power.
He wasn't destroyed, he needed time to recharge, so he escaped to the world of Tatsunoko vs Capcom, unfortunately, he didn't anticipate a few potential threats.
 * And in order to do so, he needed to screw the reality...

Yami required more power to restore himself. (Coming from the 3rd. WMG entry)
This is pretty simple: Yami was distorting the reality, and in order to do so, he needed to make some mess for it, since there wasn't so much power in his dimension. He created some sort of "Tournament" in order to obtain the energy he needed to restore himself. (Think about Z's Majin Buu) There's also a reason of why.

If Tatsunoko vs. Capcom gets a sequel, some of the new characters will come from out of the blue.
Remember Capcom vs. SNK 2, where they surprised us by putting in the likes of Kyosuke, Maki, and Ryuhaku Todoh (who took several levels in badassery from the perpetual Running Gag he was in AOF and KOF)? It actually worked, and provided a good amount of variety to cast dominated by Capcom and SNK stalwarts. Sure, I think we'd all love to see fan-favorites get in (people are still clamoring for Dante, the Samurai Pizza Cats, and Speed Racer, by the way), but adding a feasible amount of B-listers (or long-since forgotten characters) from both properties might garner some interest. Then again, it probably won't stop the Public Medium Ignorance of "What's a 'Tatsunoko'?"
 * The Marvel series also added Marrow and Shuma-Gorath, (who is rumored to come back in 3) so that wouldn't be strange.

Tatsunoko vs. Capcom is Purgatory for Frank West.
"Otis, out of film, no helicopter, zombies are too fast, not going to make it." --Frank West

(Which would mean that is there too.)

Yami is a Time Lord.

 * Cause every list needs one!

Viewtiful Joe WAS fighting alongside them.

 * Viewtiful Joe was part of it, he was just in of those dimesional portals, and than when Yami was defeated, he was sent back to his dimesion. This can also explain the heroes talking about him after he was gone and Joe saying ANOTHER dimesional portal. Add some logic, and this can be pretty sad...