First off, there's the violet eyes with dark circles under them. Sure, he could just have that resemblance in common. It's possible. But then we have the fact that another character (I think Imshael?) asked Felassen if that was what he was calling himself these days... so he had changed his name. And, presumably, after failing his master, or presuming he could trick this other person, Felassen would have taken the name Solas, Pride, as a reminder of that failure. Because he'd been proud to assume he could "win" that fight.
Now, if it's true, and he is Felassen somehow, despite Felassen being supposed-to-be-dead, there is the potential he'd betray the Inquisitor. And yet, he might once again defy his master/demon/whatever is commanding him. He also very well might be an ancient elf; Felassen never specified how old he was but it seemed like he knew too much to be an ordinary elf. Of course, he could also have learned all that he knew from chilling out with spirits and demons who answered his questions.
If dreaming alone sends Felassen to the Fade, also, and Solas is Felassen, it would suggest that he is a Dreamer, or something close to it. He used herbs to send himself to the Fade in TME, though.
If Solas is Felassen, we also have no idea whether he's gay, straight, or bi. Personally I thought that there was some evidence he and Briala shared some attraction, which was quelled by the fact Briala was with Celene and thus not acted on. But, it could just as easily have been a completely platonic relationship. Felassen could very well have been gay. There was no evidence for either that I remember. And so, he could also very easily be asexual and not interested in anyone.