My suggestion goes for Shadowrun. It's got massive flaws but some really interesting mechanics. Solely cross-platform multiplayer, where PC gamers and xbox live gamers play in the same matches.
At the start of each match you select your race, Human, Elf, Troll, Dwarf - each of which has different HP, Speed, Strength, Magic and racial ability. The team which wins the match is the team who wins 6 rounds first. At the start of each round you can use cash earned from kills, teamwork, etc to buy perks and weapons. Perks include two categories: magic and technology. You don't lose perks when you die, but you can lose your weapons, so it's better to invest in perks early until you have the build you want... It's a first person shooter, honest.
Anyway, some key magic includes things such as teleporting through walls/floors/ceilings, being able to plant a tree which heals anyone (friend or foe) underneath it, and ressurecting team mates by reducing your total magic, though if the person whom ressurected you dies then you'll bleed out, losing health constantly until someone else ressurects you or you find a tree of life to attempt to hold out under.
Key tech includes a glider that acts as you'd expect, but also lets you glide-up pretty high. Enhanced vision that lets you see enemies through walls in a radius of 70m (But other players with the perk can see whenever you use it, and you can't see enemies with the 'smoke' perk), and Wired Reflexes which lets you passively move faster, block bullets with a katana, and gives the ability to sacrifice health for a burst of speed.
Oh, and it encourages communication in a very nice way. By tapping the d-pad you send a message to your team about what you're looking at. You're looking at 2 enemies in the lobby? Press the d-pad and your team hears a message saying "Two enemies, Lobby" with a little marker for where the action is. You need help taking down a dwarf (you scrub)? Tap in a different direction and your teammates get "Need backup, One Dwarf, Helipad". Same idea applies to if you see an enemy running with the artifact (flag), or if you want your team to fortify a position you just tap and tell them to move to where you're looking.
...
But unfortunately the player population is VERY small now. So much fun, but sadly not active at all.
Edit: Someone beat me to suggesting Shadowrun? What is this madness!?