Champion idea I had, tweaked a bit with help from other people, primarily watchers of Tree Eskimo's stream.
Currently his actual theme is up in the air, according to Xypherous's Q&A thread a while back. A lot of this is built with advice based on his thread, actually.
Passive: On hitting an enemy champion with a spell or autoattack, this champion gains 5 armor and magic resist. This can stack up to 4/5/6/7 times, with one stack fading per second after two seconds of not gaining new stacks.
(Standard defense passive. This should be a melee champion, and this is crucial to how his other skills synergize.)
Skill 1: Targeted enemy is slowed by 40-60% for four seconds, fading to 0% over the duration. If this champion hits the enemy with an autoattack or spell, the current slow is increased by 10% to a maximum of the starting slow.
(Slow ability, targeted. Proccing the secondary effect won't increase the duration of the slow any, will just turn it into a static slow essentially if you keep on them.)
Skill 2: When activated, allows the use of a secondary skill for three seconds and prohibts other skills/autoattacks, with a 0.5 second cooldown. Dashes towards the target location, dealing low damage to all units in it's path. If a dash doesn't hit anyone, the skill ends.
(Blinky spam ability. The 'damaging an enemy' clause is to prevent this from being downright amazing as an escape ability. If this makes the champion too powerful from wiping out creep waves, the 'damaging an enemy' clause can be changed to 'damaging a champion' to prevent the spammability of it except on champions.)
Skill 3: On activation, your next attack deals 20%-40% bonus magic damage, plus 5% per passive stack the champion has.
(Simple low CD bursty ability. Gets better with more stacks, encouraging you to slowly build up your passive and then unleash a sudden burst at the end after a period of visible buildup. Which also has to do with his ultimate, which... )
Ultimate: The champion unleashes the build-up of energy inside of him, dealing a medium amount of damage in an AoE around himself. More damage is done per stack. A 0.8 second stun is also applied to enemies hit, rising up to a 1.5 second stun with max stacks. After using this ability, your passive stacks are removed and you are unable to gain new passive stacks for six seconds.
(Synergizes with the rest of his skill, resulting in a skillset that has a slow buildup to power, then a sudden powerful burst.)
Opinions, comments? The numbers can of course be tweaked around, I'm mostly seeing if the concept itself is good. What I'm most unsure about is the ultimate. Area effect stuns are powerful, but it's short as far as AoE ults go, and requires you to dance around the enemy team for a while before unleashing it. Slightly tanky bent in order to allow him to actually pull off his moveset, and counters exist in the form of getting away from him when he tries pulling off his combos with his slow and blinkstrike.
EDIT: Yes, I know that it's the same name as Kennen's ultimate. The name isn't really important, just having a concept for the skills and such.