Well, if Pokemon does or Mario get another rep, then Legend Of Zelda will get another rep. Because they took out Dr. Mario to even out the three games. And that's why they didn't keep Mewtwo in the game. They usually keep these three in the same amount of characters.
Not quite. While that would indeed make everything easier, it unfortunately does not work that way. Sakurai does not add/cut characters to even them with another series or balance them out. In fact, while these three series have the same number of slots in Brawl, they each have a different amount of characters: 4 (Mario), 5 (Zelda), and 6 (Pokemon). Slots don't matter, but characters do. The only reason these characters were cut is due to time.
Also, it is not
usually this way. Look at this:
SMASH 64
Super Mario - 2 slots, 2 characters
Pokemon - 2 slots, 2 characters
Legend of Zelda - 1 slot, 1 character
MELEE
Super Mario - 5 slots, 5 characters
Pokemon - 4 slots, 4 characters
Legend of Zelda - 4 slots, 5 characters
BRAWL
Super Mario - 4 slots, 4 characters
Pokemon - 4 slots, 6 characters
Legend of Zelda - 4 slots, 5 characters
As you can see, the only game in which all three series had the same number of slots was in Brawl. The number of characters also differed between each series and each game. There is no rule that each series has to be "even" with each other. So it's entirely possible that Pokemon and Mario can get new reps while Zelda doesn't.