I wish I'd had time to enter with my Ring idea. Simply having two ring slots alone would change the game mechanic quite a bit, since you could have two of the ring, or one each of two different ones. The big issue in my mind was what happens if you play a third one? I'm not sure if having two slots like that was within the competition rules, though, but it seemed like a great idea to me
But then, I'm biased...
I don't know about you nor about Elementals, but I have 8 fingers and 2 thumbs to put Rings on. I even can put more than 1 ring on each finger. So Rings would make more sense as a normal permanent than a special slot permanent.
In fantasy games, it's a common policy to have a maximum of 1 magic ring per hand. In fact, I don't think I have played a single game where you could put more than one ring in one hand. In this sense, having 2 ring slots make perfect sense.
That being, said, I'm not a big fan of having two ring slots. I don't see how it would "change the game mechanic quite a bit", other than maybe having to choose which ring to replace, which is more of an annoyance than any new fun mechanic.