I think it would have been interesting if Future Guy was mirror universe Archer, but since he was poisoned, that couldn't have happened.
But should Archer Prime have turned out to be Future Guy? I don't know that could be answered. We didn't see how it would have been written, or if done in a creative and interesting way.
Maybe archer is resurrected some how in the mirror universe and finds a way to become future guy? I like that idea!