Please also add an option, at least for human controlled nations, to leave, or at least fight becoming part of, a SOI.* Also, it could be nice, if SOI-members would get a some sort of color-scheme (vertical stripes?) in the politcal map-mode, so that it is clearly visible who belongs to whom and also so that i can overcome my desire to paint my dependants in my national colour.
Also implement options for gifts (!= trades!), please. Be it handing over lump-sums of money, or a state. Gifts may or may not increase realtions - this would be the only ´trading´-aspect of it. Give the AI some positive conditions under which it will grant some improved relations (as in: ´only if state is adjacent to any owned state, or...´, but not ´always, but not if...´), so it will be less likely be exploitable.
* For fighting to not become part of a SOI, i´d suggest, that every civ gets 20% of a GP´s influence (25% for 2nd tiers?), uncivs 10% and that they can only spend it in their own country (but in the diplo-screen, you´d click the GP to choose), counting against a chosen GP. This way, civs could, maybe, manage to avoid getting into a SOI altogether, while uncivs would be more faces with the question, whose partner they really dont want to become - but eventually, they´d become part of some sphere.
EDIT: Also, i´d like to using influence cost money (maybe depending on the size of the targets? Answer: Yes!) and an option of how much of that influence you want to utilize.
Another one: NFs enabled for foreign states for purposes of espionage (intelligence and sabotage)! Utilization of NFs could also cost money.