Aren't the Eevee evolutions representing the natural elements?I assume that since we already have evolutions that represent "fire","electricity","water","night","day","ice" and "grass",the only possible evolutions would be those that represent "sky"/Flying-type-,or "earth"/Ground-type.
If you ask me,though,I think we've got enough.At least,I'm glad that Eevee could be caught in the Trophy Garden in Diamond and Pearl,which was very relieving,seeing that in the previous games we had to wait for our Eevee and Ditto to breed together,then spend a considerable amount of time waiting for these stupid eggs to hatch...