I'd rather concentrate on the other 5 first.
The 5th Gen is one of the most well-received and popular generations of all time. A new Gen suddenly getting a character would be horrible. Unless 5th Gen atleast gets a character.
I really would rather not have 8 or 9 slots this time.
LOL. Assuming that no Pokemon gets cut. How cute.
I really doubt Sakurai is going to have 8-9 slots devoted to Pokemon series. The least I can see is 4 slots and the most would probably be 6 slots. 5 slots is the happy medium, of course. IMO, I would be satisfied with this Pokemon roster, assuming it gets 6 slots:
Pikachu = mascot Pokemon
Mewtwo = original Legendary Pokemon
Pokemon Trainer (Red with Squirtle, Ivysaur, and Charizard) = original protagonist with original starter Pokemon
Team Rocket (Jessie & James with Meowth, Arbok, and Weezing) = original villainous team with Team Rocket's original Pokemon from the first season of the anime
Jigglypuff = original random Pokemon
Zoroark or 6th gen Pokemon = current promotional Pokemon
Yeah, I know it is overloaded with 1st gen Pokemon. Whatever. But most people would be satisfied with it. It's kept Jigglypuff and added back Mewtwo while adding a couple new Pokemon at the expense of Lucario.
If people want a perfect balance of Pokemon generations with 6 slots, they have to let go of Jigglypuff to make room for a 6th gen Pokemon and accept that Mewtwo isn't coming back. Sure, it's balanced but people would be b****ing about how they finally cut Jigglypuff (therefore ruining the Original 12 tradition) and didn't add Mewtwo back. But it's more than that, it cut out Pokemon Trainer and Lucario as well.
Pikachu = 1st gen
Pichu or Johto Pokemon Trainer = 2nd gen
Hoenn Pokemon Trainer or Plusle & Minun or Deoxys = 3rd gen
Lucario = 4th gen
Zoroark or Victini or Genesect = 5th gen
??? = 6th gen