I'm not sure what I'd want to see, but logically, it seems like they'd do a ghost type next. Bug is extremely unlikely, and fighting is too (I honestly couldn't see a bipedal eevee form). I'd expect normal/dragon to be the last editions (since dragons are somewhat rare, not including pseudo legends and legends). They might introduce dual types at some point though (ground/rock, normal/flying), and it honestly seems like the flying type could be pulled off (a lot of people ((from what I've seen)) have already tried it. However, upon thinking about it, I'd like to see a steel eevee-lution. A steampunk esque look could be pulled off potentially well, but it more than likely won't happen.
This is just my personal opinion about all of this.