We can rule out poison and fighting.
15 types left to go. won't be water, grass, flying -> 12 types
I think it's safe to rule out the following types for sure. Dragon, Fairy, Normal, Bug, Ice, Ghost, Psychic (Delphox)
This leaves us at 5.
Steel, Rock, Ground, Dark, Electric
I'll rank them most likely to least.
1. Dark
2. Rock
3. Ground
4. Steel
5. Electric
While we're at it, lets look at Popplio
Normal, Fire, Grass, Flying, Water, Ground, Steel, Dark - gone
Bruxish means Psychic is likely out.
I don't think any pokemon has ever gained Bug type via evolution so I'd rule that out.
Assumption, but I have my doubts about a dragon typed starter. (barring mega)
1. Fairy
2. Ice
3. Fighting
4. Poison
5. Rock
6. Electric
7. Ghost