I am expecting Solas to be a Big Bad Boss in DA4 or DA5, and I am imagining a scene of him crawling on the ground, heavily wounded after an epic fight with the main characters. He will be murmuring something like 'lavellan' or whatever before his death.
I'm hoping it'll be more of an Abelas situation, where you can slaughter him out of hand if you wish (because choices), but you can also talk him down, convince him that the world he wants is just not a world that can ever exist, and/or convince him to lay down his burdens and go elsewhere. If a non-Lavellan, he wanders off to go into uthenera or help other ancient elves. If Lavellan, he goes off to be with her. 
I don't want him die though I am quite certain of him becoming a villain in future. And I don't think he will end in a way that what common boss like cory or archdemon ended, just a fight and die, no.
There's a difference between a villain and an antagonist.
A villain is an evil character who does evil things for the sake of being evil.
An antagonist is simply a character whose goals conflict with the protagonist. They don't have to be evil, or even bad; they just have to conflict with the protagonist about something--whether it's something they want to do that the protagonist feels they can't allow, or the protagonist tries to do something they feel they can't allow.
The devs have stressed time and time again that Solas is not evil or bad; that he is very kind, sympathetic, and cares about all living creatures (from elves to dwarves to vashoth to mages to spirits), and cares about how they're treated. He can't go the same way as them because he's not evil like them.