Well, but I think ALL boss characters should be playable in a One Piece game. It should be a given. I'm not saying the Nyaban bros, Pearl, Choo, Mr. 9, Miss Monday or Mr. 4, Gedatsu or evn Fukurou should be playable, even though some of these are actually really interesting to work with. But Krieg? Kuro? Arlong? Moriah? Lucci? Foxy? Wapol?
That should be a given by now.
Then, we could focus on secondary characters/villains. And Mr.1 was the second strongest Alabasta character, and he's had a reappearance in Impel Down, Marineford, Post-Marineford and we know 100% sure that we'll see him again somewhere in the New World. I'll agree he's not Mr. 2, but I'd say he's quite important.
As for Doublefinger, she only had one appearance and then pretty much just settled as a bar waitress and that's it. But you were just talking about how the developers have such a little pool of female character to choose from, and here you have a strong woman, with interesting abilities and quite an easy moveset to work with. Easier than Hina, easier than Vivi, easier than the Gorgon Sisters.
If we are in need for more female characters, hell, use her!