I'm facing the same issue; fire and ice seem like the best options for those reasons.
When I was leveling up my Grav/Nrg dom I too went through that same kind of situation. Originally I was thinking of picking up the Psionic Mastery app with the hopes that World of Confusion might add more control to my character to make up for some of Grav's shortcomings, but realistically the power is just flat out bad.
Personally, I'd agree that the Fire app would probably be the best choice. With Ice your overall controls might improve a little, but not by much. Going with Fire would allow you to play as a stronger AoE and Single target blaster-type with decent, reliable controls, and it would benefit both group and solo toons more overall.
I would say either Fire or, depending on your playstyle, Soul. Fire mastery does add a lot of raw damage via Fire Ball and Rain of Fire, and Rain of Fire plays nice with the AoE immob you are probably spamming a lot (assuming you've got it proc'd out).
If you like to mix it up in melee a lot, Soul Drain and Dark Consumption can add a fair bit of damage if you slot them as such, which I like to do - Soul Drain adds enough tohit without being enhanced (and you can't enhance the +damage, but you can increase the damage of the attack), and Dark Consumption gives you a hefty 20% endurance for each guy you hit so you don't really need to slot it for endmod imo. Running in and going soul drain / dark consumption gives you a decent bit of damage to start up a fight.
Fire is probably the stronger choice though.
My namesake, an Earth/Energy dom, went with the Primal Forces APP, and I havent regretted the addition of two relatively quick area attacks that thematically match my other powers.
Conserve Power came in very handy once I got these two attacks, too. These three powers work together very well if you've been building up Recharge bonuses.
They'd work rather well with Crushing Field (and its lack of -KB) to provide extra control, whereas Crushing Field would neutralise the fear-type control in Rain of Fire. Note that Energy Torrent is Knockdown, but Explosive Blast is knockback, and would cause some scatter on an immobilised group.
Considering how your primary works, I'd recommend speccing Mace Mastery for the S/L shield and Ioing to shore up that defence. You can leave control to the singularity and run around doing single-target spot control instead.
Just an idea.
so my grav/energy is lvl 40, and i've taken ice mastery for sleet, but i'm considering respecing into fire mastery for more aoe.
thoughts?
most recent 50 - psy/mm blaster