Ideally, no character would be made trash in the air like Mac again. Though, they'd probably be fairly strict in Air recovery like the other fighting game characters.
The way I would handle it is making it possible to perform combo strings with them. Where as Ryu,Ken and Terry gain complexity with their inputs, something traditional 2D fighting games are known for, I feel 3D games are known for having longer "baked in" strings. So Jin, for example, could have AAA, be his 1,2,4 input, while something like AAB would be 1,2,3, which replaces the roundhouse kick at the end with a kick that spikes the opponent downwards, as it would in Tekken, leading to floor breaks. Granted some liberties might need to be taken with this, but that's how I'd implement some of Tekken's mechanics and varied movesets.
When Sakurai mentioned he considered Heihachi, he was thinking about how to do 3D movement in a 2D game, so if he ever finds an answer he's satisfied with, we have an idea that he might have that.