Well I'd go with Earth. You're going to need some debuffing to go along with all that damage your buddies fire/fire will be doing.
My level 50 Dominators:
Madame Mindbender 50 Mind/Energy
Fly Agaric 50 Plant/Thorn
Nate Nitro 50 Fire/Psi
If this is going to be a duo more than general teaming, I would say go for Fire/ as well. Plenty of overlapping control (alternating flashfires every spawn), and once you both get to 32, six imps can bring a world of hurt to whatever you're against.
Plant and Earth would be two more excellent choices.
I'd pick a control set with an AoE stun to stack with your friend's flashfire. That'll be your primary every-spawn control starting at lvl 12.
The leaves fire/ or earth/
As fire is light on control, I'd lean toward earth/ to complement. That'll make him the primary damager and you the primary controller/debuffer.
Imps do a lot of dmg, but they're pretty fragile. Stoney would help by tanking for you.
Also, your friend will be playing in melee with HF and Cinders, so with earth you'd be the ranged guy. I'd complement that with a ranged assault set. I think you mentioned /energy... that could work. Or /psi (until 38 and PSW).
Please buff Ice Control.
Thanks for the tips. I think I'm going to go Earth/ because it's got a plethora of control as you guys have mentioned and I know Fire/ is a little lacking in outstanding control ability.
My brother and I are going to start two doms once I15 comes out, and I would like to build a dom that goes well with his. He's decided on a Fire/Fire, and I know I want to go /energy. I've been thinking about Mind/, but some of its biggest assets (aoe sleep/fear, st sleep) are going to be broken by his aoe nature. Are there any solid combos that go with /Energy that will mesh well with his Fire/Fire?