Maybe a bard-like passive, but I think not randomly spawining would be good. Unlike a lot in the wanderer archtype, Skarner has a persistent and specific purpose. Maybe the same crystal spires and spawn points, but a location-agnostic buff? Something like TT altars, or 1 minute duration buff every time he visits a captured spire, or something else. The whole spires idea doesn't actually seem all that flawed, save that if it matters then he has to be weaker than the average champion whenever he's not under that effect.
In summary, I think making the acquisition location based, but the buff location agnostic has promise.
Disclaimer: I do not play Skarner. I am merely offering up my opinions based on what I have seen with other, similar champions, my experience with the game framework, my own use of logic, and other people's opinions given to me.