There are four patterns followed in factoring Eeveelutions.
1. Only introduced in even-number gens and in gens that introduce Eevee itself.
2. Only introduced in pairs where one type is weak against the other, unless the release is stagnated due to the effects of rule 4.
3. The type of an Eeveelution must be a type that was considered ‘Special’ before the Physical/Special split (Fairy being honorarily considered Special, as that is the only category with a free ‘slot’)
4. The amount of Eeveelutions in a release conforms to the ‘Numerical Pattern of Release’, or NPR, which is found later in this post.
We will get one Eeveelution in gen 8, a Dragon type. This satisfies all 4 rules, with its stagnated parter being Sylveon. No more Eeveelutions will ever be created without corruption of one or more of these patterns.
All Eeveelution types are types that were considered Special before the Physical/Special split. And, while Fairy did not yet exist, it can be considered Special, as there is an empty space within the original pattern. Dragon is the only type left that can be made into an Eeveelution assuming current patterns remain.
Excluding the first games, Eeveelutions are introduced in only even-numbered gens, with each reveal taking 0.5 away from the previous amount of new Eeveelutions but rounding up to a whole number. This is the NPR:
Gen 2=2.0=2 new
Gen 4=1.5=2 new
Gen 6=1.0=1 new
Gen 8=0.5=1 new
Gen 10=0.0=0 new
Gen 12=-0.5=-1 new
So, in conclusion, we will get a new one in gen 8 but they will take it away in gen 12.
