1. Phoenix Wright
Capcom's famous defense attorney apparently was planned for Tatsunoko Vs. Capcom when first in development. The developers could not get beyond his finger point and "Igiari!" ("Objection!") Beyond this, if they used the word bubble the way they wanted, translating the game would unbalance everything. I guess Phoenix Wright is not meant for a fighting game.
Your Honor, I would like to bring your attention to the plot of the Ace Attorney games. All of the cases to date have been murders, which involve weapons and unique items, some of which have been used in more than one game, like Maya's Magatama. Wouldn't that be a good start of a move set? It is the defense's belief that... *SLAM* Wright would be a suitable fighting game character for Capcom.
Capcom needs to think outside the box (or word bubble) for Phoenix Wright. There are three games in his franchise (plus Apollo Justice). That is more than enough for a move set.
2. Hakushon Daimaou
Ha... ha... HAKUSHON! This beloved sneeze-activated genie was removed from the international released for Tatsunoko Vs. Capcom: Ultimate All-Stars due to some legal troubles. I honestly think Capcom can convince the copyright holders to allow the use of this guy (i.e. "This can be a promotion of the character, especially if DVDs are planned!")
I love Hakushon Daimaou (or Bob in A Bottle); the show is zany like Looney Tunes and has an interesting premise. This genie comes out of his bottle when a person sneezes, and he has to grant the sneezer a wish. Hopefully the next time I sneeze, he can grant me my wish of him being in the next Tatsunoko Vs. Capcom game.
Ingrid is a character who has been playable in a few Capcom fighting games (mainly Capcom Fighting Evolution), and she would be an easily made character.
4. Leon S. Kennedy
Jill Valentine was a character in Marvel Vs. Capcom 2: New Age of Heroes, but I think Leon deserves some time as a fighting game character. Although we have one zombie-slaughtering photojournalist (Frank West), the game can use a second zombie-killer, even though he does not carry a camera and has not covered wars, you know.
5. Speed Racer
One of the most well-known Tatsunoko franchises, Speed Racer is an anime about a race car driver in some of the wildest races known to man; Nascar, eat your heart out and run it over a couple times. He races with the Mach 5, a car with some of the craziest gadgets added. I think everyone envies the car; hell, who WOULDN'T want a car that could JUMP?
Now, the problem lies in the fact that Speed Racer is not commonly seen outside of the Mach 5. However, look at Captain Falcon in the Super Smash Bros. games; he was never seen outside of the Blue Falcon, except for a comic book, but the developers made up a nice moveset that remains well-known.
Hopefully the developers can be very creative to put Speed Racer in the next Tatsunoko vs. Capcom game.