Eh. I kind of expected as much the moment Ryan said the Warblers would be back. They’d have to cast someone new to lead the Warblers in competition (Curt just isn’t strong enough a singer, IMO), and I don’t imagine that happening.
His return is kind of a non-event to me. I don’t really want the Warblers back at all, but they have become ND’s new competition. If Blaine and Kurt aren’t back together by ep 7 (if they break up), then I see Sebastian being used as nothing more than a catalyst for them to get back together. Seblaine will never happen in canon. For one, they’d bring him back sooner than ep 7 if it were, and two, it would completely go against Blaine’s characterization. Having him give in to peer pressure in the first ep because everyone else in ND was doing the exact same thing is one thing. Having him suddenly find interest and attraction in a guy he obviously thought was ridiculously over-the-top and “didn’t like” (Blaine’s exact words), who then proceeded to blind him? Yeah, no. Not gonna happen except in fanfic. They might be friendly at this point, but that would be it.