I play a pretty mean Veigar/Leona/Nunu and a somewhat less mean Janna.
Firstly, ranked isn't a big deal, and having access to recorded stats is fun and a great measure of progress.
Here's a wonderful video giving you a neat perspective on the right attitude for ranked (not LoL but applicable): http://www.youtube.com/watch?feature=player_detailpage&v=Hz_B9H8taG0#t=573s
Secondly, you are safe learning some picks and bans and draft mode. You will find that your team will expect blitz, amumu, Shen, and malphite bans; but don't be afraid to ban twisted Fate, and impact junglers.
As for learning new spots, I would recommend: Mid: Ryze (maybe weak early, but easy to use, tanky, and can hard carry a game. Mid : Akali (low ELO pub stomper and high presence) Mid: Twisted Fate (skills shot, high skill cap, can win all lanes at level 6)
Top: Rumble (Awesome Ultimate, lane bully, few counters) Top : Shen (split pusher, game changing ultimate) Top: Nidalee (Split pusher, Poker)
Jungle: xin (Early game ganks can snowball all lanes) Jungle : Jarvan (Awesome Ulti, Tank) Jungle: Vi (Can lock down and kill a fed squishy target)
Adc: Miss Fortune (Lane bully, teamfight winning ulti) Adc: Caity ( lane bully, long range, easy to use)
Support: Thresh (day grab) Support : Sona (lane sustain, utility, AoE Stun ulti)
Most of these champs you can get the gist if in a just a few games, and because they are powerful other current meta, you can have a lot of success without much practice.
Again, I encourage you to jump into ranked, especially if you have 2 or three games in every position. Sure you might end up in Bronze V, but that is just a starting point. Also, by learning a new champ and failing in a ranked environment, you will not ever be obsessed with LP, but will have a clear measure of progress.
VerdeCreed - The Swain Train has left the station!