As for the rest of the questions posed in this thread, what spells you can make up etc...
Think of it like a "Reality by Consensus" if other characters accept and or are willing to play along with your made up RP spells etc, then it's fine to do them, just realize that settings WILL NOT CONSIDER FANON WHEN CARRYING FORTH ITS PLANS, and realize that all players will be expected adjust their skills and stories to the official story-line and mechanics when those are ready.
Meaning the spells we eventually write will be the sum of what's known and possible. Kingdoms of air will have to evaporate.
I've had some great rps based on skills or spells that were unimplemented at the time (and remain so) wherein everyone playing along had agreed to the limitations/powers of every other participant. This can work and I think regularly does looking in the "rp-storytelling" section of this forum.
The point should be to have fun which does not impede the fun others can have.
