User blog comment:KnocknickKnock/9 gates + master of the celestial realm/@comment-2.83.198.8-20131212210042/@comment-7418318-20131212212124

The thing with the snake charmer is that Ophiuchus is part of the Zodiacs, but apart from the original 12 and this 13th, there is no other constellation that's considered a Zodiac.

So any other spirit would either have to be tied to a silver key, if it's part of the 88 constellations of the Uranometria, or it'd have to be a new system, probably with keys out of another material, or even a completely different summoning medium like cards (which Mashima originally planned to use, before he switched to keys - a good decision in my opinion), charms, jewels, ect.