Sa1 did it only with 3 bosses (Egg Hornet, Chaos 4 and Chaos 6 (Which played differently with Big anyway)). I dunno, I feel it'd be a nice little progression for the series, but if it's too much, that's fine. EDIT: Another thing is, absolutely none of the classic bosses took advantage of each characters abilities, they were all based on Sonic. I realise that's our plans for the final bosses, but why leave it at that ? If having Nack and Metal as regular bosses is too much, at least have Eggman react differently to what character he faces.