I don't think dual typed eeveelution could be a good idea. Eeveelutions are supposed to be just like that, monotypes, each one representing a type.
My idea is that in the next Generations they could do a pseudo legendary eeveelution duo.
They would be much like all pseudo legendaries, appearing in their natural environment at a high level, and such Volcarona, it should be obviously difficult to envolve Eevee into something like a Celesteon or a Drakeon, but IDK what method could be used. Maybe a new one, like an event Eevee or something.
Just as Galoria said, Eevees are becoming scarce in the wild, so Eevees DNA allowed it to envolve into more powerful eeveelutions, so that they could survive. And they maneged to envolve according to their environment, and depending on that, they will envolve into Celesteon or Drakeon:
One eeveelution is Flying type, and I don't think this one should be dual typed NORMAL/FLYING, I think it should be only Flying, and much bigger than the other eeveelutions as long as being able to learn HM02 Fly.
Another one is a Dragon type eeveelution, also much bigger then the other eeveelutions. As a "counterpart" of the Flying type eeveelution, it would be able to learn HM03 Surf and HM05 Waterfall.
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