Spoilers for ep 25 in the very last paragraph (unless you don't know about the Codes yet) Up until then it's not really spoilery, just highly theoretical.
So, after a person's Geass reaches critical level, they can gain the Code from the person who gave them Geass.
What we don't know entirely is what happens to that person after giving up the Code. They can dies, yes, but do they go back to being just a regular human?
If that's the case, could the new Geass user go and give them a new Geass? Would it end up the same (it is, after all, based on a person's wish. So if their wish changed, then...) Could there potentially be a game of "swap the immortality" as each person gets a Geass, works it up, becomes immortal, and then gives it back to the other person?
Granted, this is amazingly unlikely. I have to assume that the person would be aging normally while a Geass user, not to mention the high likelihood of dying while not immortal.
...but, yeah, it just popped into my head with all this talk of 'is Lelouch alive' and if he got CC's code and such and such.