I'd destroy mankind. If I were king for a day, I'd make sure there was no -blam!-ing tomorrow.
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Better recommendation: Wipe out every stereotype and imperfection with such imperfection that it could be considered perfect harmony between all humans, despite some minor unsettling feeling when near [Unnamed human]. Basically, every single human wouldn't mind living next to their arch nemesis (if one such nemesis was of the opposite gender with everything they wanted in a [human salad?], then [DELETED COMMENT SEGMENT]), and all criminals/terrorists would be confined to their own civilization where they must survive on their own, but food is provided. In effect, the betters of our species would perfect ourselves further, while passing tomes with how that was achieved to those who are still without sci-fi-like tech. The others who would be in our tech level by this time would be able to self-manage food supplies. Too long/Failed to read: Read it, silly faesh. Alternatively: [url=https://www.youtube.com/watch?v=1zdDMgUHjFo]Human Salad[/url].