If you're going to give a player a potentially game-breaking choice, you really have to make one of the options sharing rule of the world, or the galaxy, or the universe. It's the classic temptation. It has style.
The thing is, if you offer this to your average player, 6 times out of 10 they'll choose to accept co-rule the world/galaxy/universe with the person offering the deal. And then quietly proceed to plot their assassination so they can rule independently.
Make sure you have a handy Gollum waiting in the wings ready to bite off any necessary fingers to keep things under control.
[SFX]: < doip-ding dip-deep poppity bebedooby-bing zip >
R2-D2: Okay Luke, here's the plan. We blow up the Peace Moon, or...
[SFX]: < doosquee zibuzz bebedooby-bing babioo fidooby boodit dooby doip-ding bzzt spip bloop ding >
R2-D2: We fly into the hangar near the vent, land, and close the doors. You can control all the clone troopers with the Force.
[SFX]: < doing bibuzz boodit pip-dok-kipgi doing doodle >
R2-D2: Vader is outside. Cut off the radio, and he'll never be able to override you.
[SFX]: < buzz bebedooby-bing doosquee buzz bebebang >
R2-D2: The Moon will be ours. Together, we can rule the Galaxy!
Luke: So... which was supposed to be the wrong option again?