I personally think that making nymphs more exclusive than they already are (via the Oracle or PvP Tournaments) is a waste of time. If anything, the chances for Nymphs to drop in the future won't be affected by past influence (everyday, you have a % chance for you to receive one from the oracle, no?), so putting a cap doesn't make sense, if you want to make it more rare.
Honestly, I'd rather have a whole bunch of new (but also equally rare) set of cards come out with a theme, that will hopefully be much stronger and more important than the nymphs, so as to make those exclusive from the oracle and have the nymphs be more likely won from rolls, and so on.