[quote=@Duthguy]
As for how to give humans transformations I have two idea's. The first involves genetic manipulation like what was used to create Cell to implant alien or animal DNA into humans, and the second involves someone wishing for it on the Dragonballs.
[/quote]

Sebastian mentioned the Dragonballs will be unknown at the moment, but would be cool to see humans git gud when they find all the balls later on. Or maybe the wish was already granted a long time ago which could tie into why they haven't been found or are unknown at the moment.  We can also have the Earth Guardian pull a Guru by drawing out the character's dormant power, or go the weird route like when Goku drank the Ultra Divine Water. Toriyama made things so random that I feel anything can work.