Re: You've taken over the world. Now what?
What to do after taking over the world?
Why, give it to your worst enemy, of course!
But seriously, if aliens or someone took over the world, the next step would be colonization or enslavement or one of the usuals - which means destroying/controlling military forces, consolidating power logistically, etc. If it was a Master Villain, he'd more need to manipulate existing political structures to maintain control. Lots of turning conquered/assimilated forces against potential troublemakers, forcing surrendering governments into draconian peace treaties, etc.
In the end, of course, an evil world tyrant exists to be overthrown. So, as a GM I'd develop weak points in the strategy while offering the PCs an opportunity to ally with another would-be world conquerer to throw off the shackles.