Act of Treason
Probably the cheapest Gain-Control (GC) in Standard; a definite 4-off.
Akroan Conscriptor
A Heroic-trigger GC? Meaning we can possibly gain control of two creatures off one play? Heck yes.
Catch // Release
am
The Catch is a little expensive as a GC with its 1BR cost, but Release could be a nifty resource destruction (RD) , IF we play a resource-heavy American with no planeswalker.
Hellkite Tyrant
This guy hasn’t caught on yet, and I don’t know how to make him work here. BUT, it fits with the flavor of the deck and is hard to defend against. Also, we can expect to pick up some Whips of Erebos, Bidents and Spears of Heliods with this guy…
Detention Sphere
Am
Okay, it’s a UW, so I’d definitely have to go American.  But it’s also moderately cheap, and I already have these guys in my arsenal.
Molten Primordial
Ergh. It’s a 5RR that has a GC. I don’t want to play it.
Strionic Resonator
Hah, so here’s an interesting one to include! It makes our triggered GC’s much more effective. I think I own two of this one. Also unique if I go the Hellkite Tyrant route. And I need to check if it can double-trigger a Tribute, too!
Portent of Betrayal
Takes Act of Treason, and adds a Scry1 for one colorless mana. Scrying is a great idea in a high-cost deck.
Simic Manipulator
Probably not worth it. Need to see if a GC triggers evolve, but it probably doesn’t.
Siren of the Fanged Coast
Maybe a sideboard against a non-flying deck, since it does cost 3UU. And by the time this can be played, the other guy will know how I’m playing.
Smelt-Ward Gatekeepers
A decent wall, that can give me a GC under the right conditions. Probably best in a 3-color deck, but hey, it works.
Traitorous Instinct
A 3R that gives it +2/+0.
Perplexing Chimera
For 4U, it doesn’t act so much as a GC. When it does act as a GC, it’s a permanent effect. But most of the time, it will just keep the other player wary of playing anything; and that can be a good thing.
-----------------------TO BE ADDED?--------------------------------
Domestication
A permanent GC, unless it’s big and powerful, in which case you sacrifice the enchantment. Maybe more of a sideboard card.
Goblin Electromancer
It will be tough to keep from diluting the deck with these, but they are a BR 2/2 that keeps things cheap.
Chandra, Pyromancer
Her +1 is super helpful in avoiding unwanted blockers, and her -7 basically guarantees control of the board in this deck, if we can cast three GC’s!
Oracle of Bones
They will probably pay tribute, or regret not paying it.
Barrage of Expendables
Bahahaha, gain control to sacrifice for one red, and do one damage! A must-include.
Rapid Hybridization
When it’s on my side, not only do I get to destroy their creature, but I get a 3/3 creature for one blue!
Boros Reckoner
It will probably keep the baddies at bay.
Guttersnipe
I really don’t need to explain this guy, but he would be one nice way to keep things more hasty, and a good reason to include damagey cards.
Young Pyromancer
Again, a very unique card that triggers on sorceries.
Skullcrack
Push through
Reckless Reveler
A good way to go for RD.
Turn // Burn
I like the idea of this card being controlling.’
Hypersonic Dragon
Makes the deck really mean and quick, but also kind of difficult to control and fuel.
Melek, Izzet Paragon
Can make the GC’s really cheap, but also could get frustrating to make sure that you have the advantage of surprise.
Fluxcharger
After the swap, you suddenly have a 5/1 that will devastate an open lane.