Champion Niches
ADCs:
Good ones-
Tristana: Strong identity as scaling hyper carry. Maybe nerf AOE if you are going to nerf anything
Sivir: Strong identity as the safe pick. Has Utility+Escape to fallback on, is strong in lane, provides AOE. Very good in early teamfights.
Ashe: Again a safe pick like Sivir, but falls back on CC instead of Escape.
Bad ones-
Corki: He could hold be the premier lane dominant pick but his stats aren't adjusted enough for it(or other ADCs aren't weak enough in lane). Bump up his laning stat a little more imo. It's good that he has ESC+POKE to help him harass safely
Twitch:
Basically a worse Tristana. Imo go even more hyper carry with him to differentiate. Maybe nerf his laning slightly, but bump up his scaling+AOE+TF stats. I mean.. we've all seen what a fed Twitch does to teams. He is definitely not "worse version of Tristana"
Supports:
Soraka vs Janna: Why does Janna have a higher Utility stat than Soraka???? Utility=Heals. Janna already has CC. Soraka should reign supreme here. As is, Soraka is a MUCH worse version of Janna.
Morgana vs Zilean: This is where you probably would need to implement unique traits to make a difference. Giving Zilean revives or the ability to improve mobility/esc of teammates would make him invaluable for avoiding the popping of your ADC. As it is now though, Zilean=worse Morg
Blitzcrank: As one of the supports most known for snowballing lanes, it is kind of crazy that his laning, support and burst stats aren't higher. Also Blitz shouldn't really provide Utility.. He has none in his kit. Just CC
Taric: Let him be the scaling support. Also it is weird that his TF stat is so low when he has AOE invulnerability+stun+heal+armor