I know you're baiting but I'll bite. Color Splash is OK. I don't even hate the battle system. I just vastly prefer the RPG battle system of the classics. But more than anything what the game needs is a proper story. Sticker Star had the same issue.
Paper Mario just isn't the same without the story, and the story was one of the highlights of Super Paper Mario that had me replaying it every few years. Nintendo doesn't want Paper Mario to have a story anymore, which is asinine. But now that the Mario & Luigi series is maybe dead, maybe we can see a return to form for Paper Mario.