Yea, I kind of stopped 'maining' a champion; I have favorite champions, some I play in ranked, some I don't. I've had several 'main' champions get nerfed pretty hard or reworked out of the role I played them in (can still play them, just with diminishing returns, which is why I don't play them in ranked), so basically when I'm feeling nostalgic, I just load up a normal/custom/bot game, otherwise I have a separate champion pool for ranked.
For specifics, I've pretty much been a support/mid/adc player, occasionally dabbling in jungle and top. I played Nidalee and Gragas mid, but Riot's basically pushed them into jungle at this point. Nidalee was still a good midlaner up until recent changes, but I didn't favor her for ranked games just because I felt like she's very situational; she's my favorite champion because I like her mechanics, not because I necessarily like how she functions within a team. After Gragas got reworked, I tried playing Karma mid with mixed results, but started having more success as an ADC with Jinx and Tristana, so I started playing more ADC. Tristana got nerfed and then reworked, haven't had much success on her since (hopefully that will change), but I can still play Jinx. For support, Sona was my first support though I started favoring Janna a few seasons ago and picked up Braum, those two are basically my picks, and they haven't really changed; nerfs/changes haven't pushed them too far down or anything (I'll still pick up Sona on occasion, but it's pretty rare that I need to dip that deep into my champion pool). I've had other champions I've tried to 'main' in between (Ryze, Blitzcrank, Corki, LeBlanc, Gragas support, Orianna, etc) with varying degrees of success, but these are the champions I consider myself having 'mained' in the past, some I still play in ranked (Jinx, Janna, Braum)
Nowadays I play mostly mid/support, with Morgana and Annie being my midlaners, maybe Ekko too once he's released, since i do like him a lot. Fingers crossed that he isn't OP on release (his numbers seem pretty high atm), has some ridiculous pick rate (PBE queues are incredibly backlogged right now from all the people logging on to play him; it takes 3+ hours to even get into the client), and then gets nerfed into oblivion :P. I might start playing more ADC if I get more games under my belt with the new Ashe, so I have a back up pick if Jinx is picked/banned (I enjoy playing Caitlyn, but like Nidalee, I don't really like how she fits into the game; feels like you're gambling a lot on your ability to bully the other player), in part too since Morg and Annie just got nerfed (don't think it should be too bad, but I might need to diversify a bit more, which is part of the reason I'm looking into Ekko).