I've been contemplating the later 2 of these three for a long while now...
Inuyasha, I added as a possibility, because thinking about it, I know some good moves I could use on him.
His moves would be a blend of the anime, and his moves on the PS1 fighting game.
The Vash hack (a la Trigun) would have to be over a character with a relatively large file size, as he would have an extra object, or larger primary model. This Vash hack would feature him after the series, and he would be using Wolfwood's Cross for some of his attacks. The attacks are already 85% plotted, all I really need to figure out is the balance of which gun does which (fire rates, range, damage, exc).
Beyond the Grave, since his anime is a bit less popular than Trigun, would require a little more explanation.
He is from an anime called GunGrave, which is written by the same person who wrote Trigun.
The anime is every bit as good as Trigun, but Trigun was featured on Adult Swim, so I presume that's why the larger fan base.
In any event, Grave (without giving spoilers) is a human who is strong enough honestly, to blow Vash to kingdom come (minus the Angel Arm). With his twin handguns, and armed coffin on his back, it would be easy enough to create a moveset based off of the anime, and the second PS2 Game "GunGrave: Overdose".
This one would, however, be the most difficult hack to pull off, due to the physics of the coffin animations, and numerous projectile attacks.
On that note, I only know a substandard amount of making a PSA: I'd need a volunteer for whatever seems like popular vote: so if any of you would want to help out, please post. I can do the animations, and may be willing to learn how to work on models more... My main thing about it though, would be designing the attacks and coordinating the animations, it's all I really have have time for right now, as I have numerous site updates I still must accomplish.
If I don't have 1 or 2 people to help with the PSAs and such for the highest vote, but sources for the second, I'd likely choose that one, just to get one done sooner.