CONCEPT: A pure flying type pokemon, that sort of looks like a cool hurricane or something. It'd learn really strong and cool moves such as Aero Blast, Ice Beam, Surf, etc, because it's based off of a hurricane.
DESCRIPTION: It's ability would be that it's stats are modified if it's hit by certain elemental moves. Fire and Rock would lower it's speed and special attack one stage. (it's 2x weak to fire, 2x weak to rock) Ice would raise it's speed and special attack one stage. (it's 1x effected by ice). Water and Electric would also power it up like Ice does (it's 1x effected by electric). Rain and Sun would also power up/weaken it (but non sandstorm and hail)
WHY IT'S COOL: I've always wanted a Tornado pokemon since the good old days of RBY. A hurricane could be a really cool idea. It's be mainly defensive, allowing it to counter stuff like Garchomp, Lucario and Heracross pretty easily if it can avoid Stone Edge. However, being hit with Water, Electric or Ice would power up the hurricane and make it a serious threat. Recover (not roost, that'd be too good) would make it a popular wall on many teams, but sandstorm would make it offensively retarded.
It'd have plenty of counters, such as Tyranitar, so I don't think it'd be too broken. It'd also keep freaking Revenanhk in line (seriously guys, it's pretty cheap) with it's Fighting resistance, and lack of Ghost weakness. All in all, I think that it's a neat concept, and it shouldn't be too hard to do.
You're breaking several rules and you're jumping way, way ahead. We're not talking about the types, we're not talking about all the super sweet moves it will learn, we're not talking about counters because we don't even know that yet. We're only talking about the roles the pokemon will play. Read the first post on this thread.