A long time ago in a galaxy far far away.
You could set it – scaled down a bit, but we don't see more than a few planets anyway – somewhere in Heaven's Reach, really.
Alternatively, you could cast the Jedi as Sidereals and their mortal pupils, who sealed away the concept of Exaltation millennia ago when it threatened to destroy the galaxy. Now, they preach a philosophy of detachment and balance to dissuade their return, but Gold Faction Qui Gon is unable to resist tutoring the young boy destined to become a Solar, Anakin Skywalker.
Unfortunately, Palpatine's a Deathlord who can evade the Sidereal prophecies, corrupt Anakin, and see him be Exalted as a sort of Solar – namely, an Abyssal. He now runs the Empire from atop his seat, and employs corrupted Sidereals and his servant Darth Vader to snuff out the slowly-returning Exaltations before they can gather momentum. The Force is Essence, obviously. If you want you can have Mandalorians be Terrestrials, but I am heartily sick of Terrestrial-wank.
...not sure
why you'd go in for that, or what in the world the story would be about. Lilo and Stitch is a story about family crossing barriers and withstanding hardships. Power and consequence and heroism and action don't really come into it. If you give Lilo a Solar Exaltation she'll, I dunno, make nicer sandwiches for the fish, or something. If you decide that Stitch is a Lunar, practically nothing changes because he's already a fiendish overpowered murderbot who learns the meaning of family.
And out of morbid curiosity, let's see you do 40K.
This really depends on how extensive you want to make the changes. If you want it to be identical to canon-40k on the surface, it's simplest to replace ancient Terra with forgotten Creation, and cast the Terran warlords of the Age of Strife as warring Solars, whose conflict would have torn the solar system apart if the Emperor hadn't emerged victorious. The greatest among them, he defeated all his peers and trapped their divine souls in the Golden Throne, uniting Terra under his singular banner with the aid of the Sidereal Malcador. When it became clear that even his awesome skill wasn't enough to run the Empire of Humanity on his own, he tried taking twenty of these Exaltations and fashioning vessels around them, to create his Primarch-children.
Sidereals lurk throughout the Inquisition, split into Radical and Puritan rather than Gold and Bronze. The Chaos Gods are the last, mad Primordials, trapped in the groaning prison of the Immaterium and tempting mortals into their service with promises of akuma-power. Machine-Spirits are literally machine spirits, and tech-priests work on more than religious delusion. I don't really see much space for Lunars, though Terrestrials could potentially be fused with Space Marines as a whole, "inheriting" their elemental blood directly from their Primarch by surviving infusions of gene-seed.
The protagonist is presumably a normal person in the Imperium of Man who receives a Solar Exaltation as the Golden Throne creaks and cracks open.
