Well, Mr. 1 and Vivi have a big enough move pool to make them battle characters. Vivi was in Grand Battle / Adventure and Unlimited Adventure…and Mr. 1 was apparently in the wonderswan games from what I've heard.
I think Law, Kid, and Marco all have shown a decent amount for a moveset...I mean they put Sasuke into Jump Ultimate Stars even though the manga had just finished the Grass Country arc and Sasuke only showed 2 attacks. They made those his specials and filled in the standard moves.
Generally though, I agree with you....Doflamingo is as important as you say...but just hasn't shown nearly enough...at least with Garp and Sengoku you have some potential for specials and could fill in the standard attacks as regular pucnhes or whatever given that they both seem to fight with their fists rather than any sort of weapon. Similarly they just add some kind of haki sword attack for Shanks special and fill in standard sword attacks. Same goes for Law, Kid, Marco, and Inazuma....Vivi and Mr. 1, I think have enough for specials and standard attacks.
Shanks is tricky....he's very important to the story and we technically know he fights with a sword. He hasn't really shown a single attack yet, but they've included him in pretty much all One Piece games as a playable character, so I don't really see that changing especially given the role he plays at the end of the war.
Sengoku...I guess, they could POSSIBLY add him...but we know so little about his power. I guess he might fall into the same area as Shanks though because of how important he is.
Doflamingo, I don't think will be playable simply because we haven't seen him attacks someone even once. He SOMEHOW sliced off Oars Jr's leg...and can control people like puppets. I think at this point, it makes him a support. I mean, I think Oda's saving his power because he'll be a major arc villain in the future...maybe even the center of an entire saga (akin to Lucci, Eneru, Crocodile)...I think Kid, Blackbeard, Doflamingo, and Kaidou will all be major villains in the future alongside others we haven't seen / heard of yet.