Re: Wario Ware and Paper Mario
This will highly hinge on how popular these series would have been without the Wario and Mario brand names. I’ll assume the best case scenario where the franchises were as popular as what actually happened. For the Wario Ware and Paper Mario series, I guess that they would probably be passed over in Brawl given that Pikmin was the only new and relevant non-Mario series added. In the case of SSBFor, I think that there could have been a chance for both franchises if there was a gameplay hook for Sakurai as they had multiple games under their belt, and we had multiple new franchises in For. Best case scenario, we’d have Ashley/Mona and the Sticker Star protagonist, who would be added at the expense of Bowser Jr., Jigglypuff, and potentially Duck Hunt or R.O.B. I assume Ultimate would pass them over because of Ultimate’s focus on requested fighters base game and third party DLC content.
Re: Sheik
I’m truly 50-50 on Sheik’s odds of returning. On the one hand, she has been present for four mainline SSB games, which isn’t really any different from other staples like Ness, Captain Falcon, Falco, and Mr. Game & Watch. On the other hand, I feel like her additions in Brawl and For hinge on Zelda’s original gimmick included in Melee and Brawl and intended in For. In the case of For, it is possible that Sheik was pretty far in development before the split needed to occur (Being a Melee veteran probably helped, but I don’t think that’s the whole story). Because of this, I’m really unsure where she’ll be in terms of priority.
Regarding Impa directly replacing Sheik, I neither expect nor want that to happen as I find it unnessary to add a replacement character instead of just making a (semi-)clone or echo. If Impa was to join, I assume she would have a different moveset from what Sheik has.