Narrowing my champion pool

I realized the other day that I was allowing myself to get spread too thin across a number of champions. My mechanical skills are "ok" but I need to get "good" with a narrow selection of champions before branching out. So I've set myself a goal of establishing two champions at every position that I feel like I should try to be "good" at.

ADC:
Vayne: A fairly straightforward champion. She has a decent escape with a super low cooldown. Her condemn gives her a second type of escape and she has a reasonably straight forward build path.
Urgot: Builds tanky and more of a "caster ADC" rather than an auto-attacker.

Mid:
Kassadin: I've gotten fairly good with him. He's naturally resistant to magic attacks and has fairly high burst with good AP scaling. He's ridiculously mobile with his ult on a short cooldown and has a massive aoe stun and great mana sustain.
Diana: I've always liked her. They rebuffed her back to where she's good again. She also can serve as a jungler which allows me to further consolidate my champion list. Can build tanky or full burst.

Top:
Yorick: I've always liked him. Has a natural tankiness and creates annoying obstacles for a number of champions. Doesn't get played a lot so should make some people uncomfortable.
???: Looking for suggestions. I've tried Elise and I'm not good with her. Malphite is permabanned so a bad choice.

Jungle:
Hecarim: I love him! He's, by far, my best champion at the moment (though close with Sona, I suppose). He has great ganks. Can build tanky and DPS (often at the same time) and has a powerful ult that will change the face of a game. I really want to play a game sometime with Singed top and Malphite mid so I can go full hybrid damage with him and see how it works. Really a fun champion.
???: I've played around with Nocturne but I'm not sold. I'm good with Amumu but he's a permaban. For now this slot will be Diana but would like a dedicated jungler to learn.

Support:
Sona: My best / second best champ. I love the poke from her combined with the sustain. She has a number of different builds allowing me to stay full support or transition to dealing damage. I like building mana manipulator on her right out of the gate because it combines well with an Ezreal or Miss Fortune who tend to be mana hogs early.
Taric (though maybe should be Nunu): Both these supports are good. Taric offers a stun and maybe could be my top laner (giving that tack some appeal). Nunu offers good harass and an awesome buff as well as self-sustain (and could, theoretically, be my jungler).

Any thoughts or suggestions would be appreciated.