With Paper Mario, you just take various moves he uses from his 3 (soon to be 4) Paper Mario games. He could use him hammer (probably for tilts and smash attacks), turn into paper air planes/ships/rolls, use jump-based attacks, star power attacks, etc. Literally you just have to play the games, and the moveset mostly writes itself.
Bowser Jr. you could just give him a paint brush and go crazy if you want an easy path. Although I will admit this one would require more thinking... which is why I am hesitant about Bowser Jr.
How do you know=??? I heard he farts out fireballs...
The swimming recovery idea made me chuckle. That would be a "unique" jump, but I'm not sure about it as a up B.
As for the tennis racket, Peach already uses a tennis racket for her forward smash. Although the tennis ball idea is a pretty good one, I will give you that. However, it could wind up being too similar to Diddy Kong's bananas.
Oh gosh, now it is turning into a Mario spin-off feast. Eoww.
This would probably be my reaction to seeing that if Waluigi was playable!
Also why do some of you want 3 Mother characters=??? As a huge Mother fan, 2 is fine. Ninten is basically just an early incarnation of Ness (although an alternate outfit would be nice), The Masked Man is a clone of Lucas (again, an alternate outfit would rock), and the main characters of Mother 2 and 3 are already playable. The rest can be assist trophy characters, NPCs, and bosses. The series isn't that big. Mother is really "set" so far in terms of characters.
Now we just need a quality stage in a future smash bros game. We've gotten two from Mother 2, and one from Mother 3, so now we need one from Mother 1. That and maybe another item. Teddy Bear anyone=???
(I would love to see wrapped present bosses as common adventure/story mode enemies, lol!)