No. No more Pokemon trainers. That idea's been brought up a bunch of times, let it die.
And as Andrew said (I think it was Andrew?), I really really doubt that Red will change into the other trainers from other generations. Maybe colour wise, he will have their colours but that's it, and he especially won't change into a female. Sakurai already referred to Red as a he several times, so it's clear that Pokemon Trainer is a boy and will only be a boy. If Red was going to be able to change into a girl, then it would have been mentioned when he was first shown, and the girl would've been a part of the profile picture.
For those using the name "Pokemon Trainer" as an argument for why there will be alternate trainers, please stop. I beg of you. I'm going to set things straight right now. To be honest with you, Red is American canon. In the Japanese version of Pokemon Gold/Silver/Crystal and in Pokemon Stadium 2, "Red" is simply known as Boy Trainer ???. That's right. He has no official name. Beginning in Ruby/Sapphire, main characters began to get real names, with Yuuki and Haruka (known as Brendan and May in English) and Kouki and Hikari (Lucas and Dawn) in Diamond and Pearl.
So Sakurai really is just sticking to what the Japanese know. He is simply calling him Pokemon Trainer because unlike the R/S and D/P heroes, he does not have an official name. His official name might very well be Satoshi, considering he's named after the original creator and the rival Shigeru is named after Shigeru Miyamoto. But that would bring them into anime territory, so it's a lot easier to refer to him as Pokemon Trainer, and we know the Japanese did want Red because Satoshi was the third most wanted character Pokemon-wise for Melee.
So yeah...even though Red doesn't fight himself, Sakurai made it clear that he is Pokemon Trainer, and the FR/LG one at that, so I doubt Red will actually change into any other trainer, but will simply take on their colours at least.