i think it'd be best to state that if it's not within a knowledge a Warriors cat could have, then it wouldn't be allowed!
e.g. if the campaign had villages and dragons and magic, that isn't something a Clan cat would have any sort of understanding of so it wouldn't occur in their dreams @
hermeswind:
if the character was a kittypet, there is some more flexibility but i would still be careful of going toooo elaborate with the dreams since cat understanding is limited