For Wild Mass Guessing concerning specific games in this sub-series, see the below links: Wild Mass Guessing concerning the series as a whole.
Mario and Luigi will eventually team up with Donkey Kong.Bowser and King K. Rool will also join forces to fight the trio.
Starlow is mean to Luigi because she secretly has a crush on him.Starlow finds Luigi's shy, awkward demeanor to be endearing, and treats him like a loser as a form of romantic teasing.
The next game will have Peach teamed up with Starlow as a playable character.As a single character, I mean. We know from a few games that Peach has SOME platforming skills, and the last two games reveal that wish power with Starlow is powerful enough to lift an unconscious Bowser and toss him aside, and later stop Bowser from using the Dream Stone and then destroy it! And with Peach being playable in ANOTHER recent game, it's only inevitable. Plus this gives another sort of justification for giant battles: Peach can just wish for a giant robot or for someone to become giant or something!
One of the next games will feature the Massif BrosFawful was super popular and got promoted to Big Bad before being killed off, Popple and Kylie Koopa got brought back because they were popular and the series really likes bringing back Ensemble Darkhorses, so it seems kind of logical that the most popular non villain characters of Dream Team are likely to end up in one or both of the next two games in the franchise. And their jobs as tour guides makes it even easier to explain why the heck they might end up on the other side of the world for no apparent reason.
Bowser suffered brain damage during Superstar SagaEarly in the game, Bowser gets amnesia after being shot out of a cannon, becoming Popple's apprentice "Rookie". He regains his memory in the middle of a fall from a great height, right before he hits the ground and is knocked unconscious. It's highly likely that one or both of these events gave him brain damage, resulting in his less-than-bright behavior during the rest of the series (including his failure to recognize his own past self in Partners in Time). He may be recovering as of Dream Team, going by his increased Genre Savvy tendencies and his recollection of Luigi's name at the very end.