Eevee's evolutions are triggered by environmental changes and exposure to elements:
Vaporeon - water
Flareon - fire
Jolteon - electricity
Espeon - day/sunlight
Umbreon - night/moonlight
Leafeon - plants/forest
Glaceon - snow/ice
So, what else can it work with?
Ground-type Eevee - desert
Rock/Steel-type Eevee - mountains/caves
Poison-type Eevee - swamp/toxic land
Ghost-type Eevee - graveyard/burial chamber
The first two seem more likely.
Vaporeon - water
Flareon - fire
Jolteon - electricity
Espeon - day/sunlight
Umbreon - night/moonlight
Leafeon - plants/forest
Glaceon - snow/ice
So, what else can it work with?
Ground-type Eevee - desert
Rock/Steel-type Eevee - mountains/caves
Poison-type Eevee - swamp/toxic land
Ghost-type Eevee - graveyard/burial chamber
The first two seem more likely.