Tatanga will be the Big Bad.
The final boss will be an Evil Knockoff of Mario.Past final bosses of the series: a nigh-invincible star-powered Bowser, Princess Peach possessed by an evil demon queen, and an unholy fusion of Luigi and the BigBad. Now look back at Super Paper Mario: who are the heroes of the Light Prognosticus? The red-clad mustachioed plumber is next on the list. Obviously, Mario can't fight himself, so Bowser will probably use magic to create a clone that can make him meet his match (Metal Mario, maybe?).
To make up for not having partners this time around, characters from previous games will have cameosAnd if there's an arena level, at one point Troopa Jr., Lord Crump, and O'Chunks will team up against Mario 3-on-1.
There is a reason Bowser has no dialogue...Chances are the crown he is wearing doesn't allow him to speak, which comes at the cost of reviving him as giant cardboard version of himself. The Royal Stickers drove their wearers to madness, Bowser's crown made him an Empty Shell.
Sticker Star is an in-universe children's storybook.It certainly explains the inconsistancies with the rest of the series and why everyone is apparently actually made of paper.
In the event future Paper Mario games continue down the "main games/Mario's world-only" criteria for characters, characters from newer games will appear.For sake of example...