I've always wanted to play a Gnoll, but never got the chance to.
Swordmages are fun, you'll likely be jumping all over the place and act as both defenders and controllers.
The fighter builds are good too. I tried the two-handed build (can't remember the name, but I think it was shown in PHB2.) and it was really great, a bit less strong than a double-wielding Ranger, but the ability to mark more than makes up for it. (you'll still be doing a lot of damage.)
Assassins look fun, as do Monks. I haven't seen nor tried the Brawler build for the fighter which was introduced with Dark Sun, but that looks cool as well.
As for a Leader, maybe a Warlord? I played the Bravura Warlord, and it could be either a god-send if you have a few melee allies and a DM that roleplays enemies, or very bad if you have lots of ranged allies and a DM that plays "optimally".
If you're looking for something completely different, try a Hybrid build. There's a few good examples on the D&D 4e forums of awesome builds.