Citer
Since the dragon type is pretty imbalanced at the moment (having only one resist and being mostly exclusive to some of the fastest and most powerful Pokemon in the game), I think it's somewhat likely that they will introduce a counterpart to the dragon type, one that is equal standing in terms of power.
With the etymology for "Sylveon" likely being true to the case presented by >>11341613, and Xerneas being a representative of a mythological woodland guardian, I think it's likely that a new type, representing sprites and spirits of that kind, will be introduced, and given to Xerneas as a counterpart to the somewhat likely Yveltal dragon typing.
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sylvan (adj.)
"of the woods," 1570s; earlier as a noun (1560s), "deity of the woods," from Middle French sylvain, from Latin silvanus "pertaining to wood or forest" (originally only in silvanae "goddesses of the woods"), from silva "wood, forest, grove," of unknown origin.
(Taken from the online etymology dictionary)
Seeing as the English name may come from some mythical being that resides in the woods, and the German name may come from their word for faerie, this Nymph-type business seems to be getting more and more legitimate...