I agree with Bird Person. While my friend and I were playing Melee on Brinstar Depths, I began to think of the Melty Molten Galaxy and said, "They should make a Mario Galaxy stage for brawl!". I mean, think about it. It could be like the side-scrolling stages, too. When you jump close to another planet, you get caught in its gravitational pull or something. And Star Bits could rain from the sky and be picked up and thrown at opponents.
For a character suggestion, Luigi. Not Luigi with Mario's moves, but Luigi. He could have his traits from Galaxy (jumps higher, farther, runs faster, stops harder) and other Mario games. Top it all off with new moves to make a completely original Luigi. His Final Smash could be, like, Mr. L or something where all his attacks are darkness-oriented and twice as powerful.
An assist trophy idea is the Fighting Polygon Team from SSB. We haven't seen them in ages. I guess they could all do some kind of team-based attack to do major damage to your opponents.
I have two suggestions for items:
Floro Sprout- From Super Paper Mario, it could briefly turn your character into a Lv. 9 CPU while you sit back and watch the Brawl for a bit.
Sport Stick- I thought this one up. When you pick it up, it turns into a random sports object like a tennis racket, ping-pong paddle, bowling ball, etc.
On the topic of Waluigi, he is not needed. He was simply created as an extra fill-in for Party and Sports games.