My thoughts for Seviper were to combine him with Toxicroak G. I'm working on a list. I think he has potential as a partner for Toxicroak, since he still can hit SP's for considerable damage whereas Toxicroak can't.
Lostgar will actually become way better after rotation, imo. EVERY deck is going to take a hit in speed. Lostgar can still function through Mew though. It can start lost zoning people as soon as Turn 2, and hardly any deck will be able to donk because of Rare Candy's new rules, and the most likely exclusion of BTS.