The thing is that it's one piece, they already have faced a mind controlling guy, a guy who manifest memories, a guy who draws their enemies, and retellings of the story. If you go ahead and desing another Barroque works for the crew to fight them, you take your chances on creating 10+ characters that might not be well recived, and might end just using for one game.
Come on, next should be either Super Smash Musou or Weekly Shonen Musou!
Super smash for the variability of the characters and worlds that they could use, and WSJ for that plus the fact that these kind of fighters and their elaborate and cinematic attacks don't mesh well with fignthing or adventure games, of stoping the action for you or worse for a bunch of people, just to see the eleventh cutscene of a kame hame ha.
Instead here, it's flashy and impactful enough to be interesting, but while takes time, it's usualy a series of steps of the attack, as different animation and hits.
The bad part is, that bosses feel just like fat mooks, they either overpower you instantly, or you beat them just like you beat the thousandth pirate/zombie/chinesePeasant/Lizalfo. I think that what they did to the big bosses (Dodongo, Ghoma, Agharog, Mandala, the Imprisioned and Ganon) is more interesting, and should be translated to the multiple spinoff that they make.
Like the Skypea Scenario, you can't start with Luffy (Who in the story mode gets +defense against Enel) you go, and capture areas defeating the priest, guerrillas, and the bunch of enemies, until Enel comes for you, and if you face him with high enough levels you beat him and goes into retreat for a while like in the others, but if you are in the levels who are apropiate for a player in this stage, you start to get zapped from afar, until you are defeated or escape the range, that goes on until you activate the event for Luffy who then starts the final battle with Enel, who after a short skirmish, handicaps you with the giant gold ball and keeps escpaing.
The Opinion of someone who watched a bunch of let's play of Pirate Warriors and Hyrule Warriors, and only played DW3 and 8.