I think that giving people a domain like power for each sphere sounds like a good idea, simply because right now, each sphere is a little bland, though it would be important to figure out how powerful each power is.
Better than +2 to a certain skill, at least.
Maybe the equivalent of Air? I.e. Rebuke/Control/Bolster/Turn/Destroy things that oppose it? What is the equivalent of that though?
Possibly only for the archetype significant one?
Hmmmm.
Edit:I played in a homebrew AD&D game where were were a bunch of not-south sea islanders fighting against the invading not-dutch colonists.
My character was a halfling Monk/Priest (the kit that turned the Priest into an unarmed fighter, I think) who worshiped the god of everything and nothing. Whenever I rolled unarmed (since AD&D had the best martial arts chart), it always landed on either head kicks (frankly impossible for a halfling) or crotch shots, so the DM just said that everything was a hit to the crotch whenever I rolled.
Fighting giant spiders? Crotch Shot.
Fighting Stone Giants? Crotch Shot.
Fighting a Giant Squid? Crotch Shot.
The best part was is that my character would semi-randomly disappear and reappear, since I had to work while the game was going on, so I was the magically vanishing halfling who would punch everyone in the crotch and then vanish into the mists.