...Law? Really? I get that he's popular (and that's probably all the justification that they need, really), but out of all of the One Piece characters that they could have chosen, he's probably one of the least interesting for me.
I like the My Hero Academia choices, and Buu depends on which version of Buu it is. I would have been perfectly fine without two new Bleach characters, though. Grimmjow never really stood out to me (he seemed like a less entertaining version of Kenpachi), and Hitsugaya felt shoe-horned into the plot.
I REALLY don't like the Uchihas in Naruto, so I can think of a lot of characters that I would have preferred over Madara. (Hinata, Lee, and Gai for starters)
Even if they just limited it to series that are already in the game, where are the JoJo's Bizarre Adventure characters? Bare minimum, you'd think that either Jonathan or Joseph would have made the cut, since they were in the last game, and Bandai-Namco made a JoJo arena brawler, so there are about fifty different options to choose from there. What about Raoh from Fist of the North Star? I'd pay good money for him.
I'm neutral on Kaiba and Biscuit, assuming that Biscuit is the one from Hunter x Hunter and not a different character with the same name.
I don't have the game (mainly because I don't know if my computer can run it, and from what I've seen, I don't know if I'd pay full price for it), but this DLC roster seems lacking in my eyes.