From whats been shown so far Bowser teams up with Antasma to defeat Mario and Luigi, and in one of the earlier battles, it's shown that Antasma does "possess" Bowser, although Bowser is still in full control from the looks of things, Antasma is just powering him up.

Oh, geez. Well hopefully it's played for laughs. Kind of like a "Well, everyone has done this in the past, so let's give it a go!"
And if it happens early in the story, hopefully it'll be a clever way to use it early and make way for some new stuff.

From whats been shown, a lot of it is used as a means to explain how Bowser is able to slip in and out of the dream world, which makes sense seeing as he'd have no way of doing this on his own. Either way, I'm fine with Bowser teaming up with the bad guy this time. If I'm not mistaken he's tried to help the Bros in every go so far (SS he takes them to the Beanbean kingdom, in PiT his younger self save the babies, and he helped them unintentionally in BiS), so this game is breaking tradition by having him being a flat out villain (I kinda hope he helps you at the end to keep the tradition going though).