seeing tropius and skarmory with a pre-evolution would be awesome :D
it is possible for more eeveelutions, because in every generation with an even number, 2 new evo's were introduced:
Gen.II; Espeon & Umbreon
Gen.IV; Leafeon & Glaceon
You see that in gen II, 1 eeveelution is super effective against the other (Dark - Psychic) and in Gen IV, the same thing happened ( Ice - Grass)
And the first one, which is odd.
Gen I: Eevee & Flareon/Jolteon/Vaporeon
..
but anyhow
2 instances ARE NOT A PATTERN.
Like, how do you know they don't "wait" always +1 generation in between?
1- yes
2- yes
3- no
4- yes
5- no
6- no
7- yes
8- no
9- no
10-no
11-yes
...
Of course this is just as ridiculous and gamefreak probably decides whetehr they make an eeveelution on actual reasons, not because of a pattern.
Things they would include in such a consideration would be like,
-what kind of generation it is (evolutions or reboot)
-if they have good ideas for more
-if they even want to make more, or whether Eevee fulfilled its purpose already with the 7.
Things like that.
Whether a generation is even, I assure you, doesn't have any direct relevance to the matter.
As for "one of duo weak against other".. its also irrelevant.
For example Umbreon was also immune against Espeon, while Glaceon doesn't even resist Leafeon.
Its simply because if you pick 2 random types, there is a high chance one will be weak to the other.