Who do you want back

[Rookie]

I always wanted to see Popple from Superstar Saga return, and it seems he does in Dream Team

 

[Revin]

i'd actually like to see lubba return

 

[Im Yoshi]

The Yoshi from ttyd

 

[Dorayaki]

IMO:

Gene & Plum: there should be recurring sport aces in Mario World.
Goombella: similarly a nice RPG and goomba tribe representive.
Syrup: nice female enemy choice in sport spinoffs.

I want Waluigi back.

Both offtopic and unclear case. Many main characters didn't get to Kart 7 and Tennis Open which are handhelds.

 

[Fawfulthegreat64]

There are MANY characters I want to see more of. I'll make a list:
Fawful
Cackletta
Dimentio
Midbus
Dark Bowser
Count Bleck
Grodus
Doopliss
Shadow Queen
Mr. L
Lord Crump
Shadow Sirens
All Paper Mario partners (SPM pixls included)
Popple (in Dream Team, but that doesn't necessarily mean he'll start appearing more)
Antasma (New, but awesome.)
Prince Peasley
Prince Dreambert
Wart
Waluigi (appears much more than the others, but I want him in a main game or RPG)
Tatanga
O' Chunks
Mimi
Rookie (technically Bowser, but same with Mr. L, something could happen to make him return)
Dark Star
Dark Fawful
Bowletta
Super Dimentio
and Many, Many more.

I don't mean just for one game either. Most of these I want to be regulars, which won't happen but I'd love it.

 

[Dorayaki]

There are MANY characters I want to see more of. I'll make a list:
I don't mean just for one game either. Most of these I want to be regulars, which won't happen but I'd love it.

Depends on which side. Though this is indeed a dream discussion thread, realistically speaking, the amount of current recurring Mario characters are very enough. If you think some characters should be treated same as Peach or Luigi, it wouldn't be more than 2 or 3.

Some suggestions like RPG bosses are very unlikely since they won't return in most conditions and you can't reintroduce them in another story outside the original game.