Each time there's been a new set of Eeveelutions there's been a new method of obtaining them (stones, happiness/timing, location) If there's going to be more I wonder how they'd evolve this time round? The only other way would be trading like Haunter - Gengar, unless they decide to bring gender into account.
Move based evolution!! Let there be a movetutor that gives your Eevee either a Dragon-type move, or a Ghost-type move, and level it up, and boom.. Eeveelution #8 and #9..
Gen II would like a word with you...
Gen I - 151
Gen II - 100 added
Gen III - 135 added
Gen IV - 107 added
Gen V - 156 added
I really doubt they'll add another 150 odd, two gens in a row - expect nearer 100.
That's a shame though.. Sinnoh felt empty because of the shallow amount of new families.. And lord did I feel annoyed by having to see Geodude, Ponyta, Zubat, Onix and what not half of the time.. With Johto, I thought it was okay, there were more new families, and only some got additions, but those were baby pokemon in most cases. Yes, Steelix, Scizor, Slowking, Politoed, Umbreon, Espeon, and Kingdra do exist as well.. Shoot, forgot Bellossom, Porygon2 and Hitmontop, not to mention Blissey.. Oh well.. Chansey was dead to me in GenII because of the Special-split..