Currently I believe only the four primordial domains, plant, and animal actually affect your summoning theme, it would be splendid if more had this. Just looking at the list of domains, these jump out as easy to correlate with an existing theme. I have not tested all of these, but I am fairly certain that the first five do not change themes. Some of these themes are pretty strong, but the domains that match up (Law, for example) aren't gamechanging in their own right, near as I can tell. Rather than buffing them, which I do think some domains need, I think this would be an easy(?????) boost to cleric diversity.
Chaos
Death
Evil
Good
Law
Illusion
Darkness
Spider
Chaos
Death
Evil
Good
Law
Illusion
Darkness
Spider