After some careful research and consideration, and based on Rowlet's Grass/Flying type, I believe the leftover two starters will have types focused on a secondary counter cycle of types as to match with Rowlet, even though its not too obvious at first sight. What do I speak of you say? Easy, I speculate that Litten and Popplio will be these types in the end:
Litten's evolutions:
Fire/Rock
Popplio's evolutions:
Water/Fighting
With Rowlet being a
Grass/Flying from the start, you get the following chain: Grass/Flying -> Water/Fighting -> Fire/Rock -> Grass/Flying, repeat cycle.
Why
Fire/Rock for Litten's evos you ask? I put this guess mostly due to its attack animations. As one can see...whenever it attacks, its fur rises up in the middle, forming what seems to be miniature volcano-like shapes, especially with the red tip, which could be representing lava. In essence, you could say that Litten's fur always "erupts" when he attacks. In addition, Alola is a tropical region based on Hawaii, and what is Hawaii known for? That's right, volcanoes. Adding to this, the marks on Litten's forehead, as well the shape of his eyes is speculated to be a alchemic symbol for Sulfur, alternatively known as
brimstone, a substance that can be found around volcanic areas. Look it up.
https://en.wikipedia.org/wiki/Sulfur
The black fur in return could/may hint to obsidian, which is black glass made of hardened/crystalized magma. In addition, volcanoes always erupt volcanic smoke/gas, which in turn has various gases in them including
sulfur dioxide, a toxic gas active by volcanic activity. As result, a
Fire/Rock typing would indicate a defensive, but mid fast/attacking type. Also if this kitten evolves into a tiger, these beasts are known to be very enduring and powerful by nature, and are also the largest feline around. Combined with a rock hard defensive hide, this cat is not one to mess with. That said, however, sulfur could end up being the key for Litten to become a
Fire/Poison type, but I'm yet not too convinced. Its way too easy and predictable. I mean, who expected Fennekin to become a Fire/Psychic type in the end? I didn't expect it.
In addition, its Dex entry, as well its personality and behavior seem to hint to volcanic themes:
"
Logical but also passionate, Litten always remains coolheaded and doesn't show its emotions on the surface." -
Dormant volcano.
"
When the time comes for Litten to shed its old fur, it all burns up in a glorious blaze." -
Erupting volcano.
For Popplio's possible
Water/Fighting type, however, I pulled out the Fighting type mainly because of one thing only; Popplio's acrobatic, swift fighting style. Fighters, especially fast ones, can be acrobatic, and combined with water, which on itself is fluid, fast and agile, you're getting a pretty swift, fast fighter, but one with low defenses. His art already shows a pretty good use of how he tends to use his bubbles in a fight. Its easy to imagine that he could form bubbles around its hands as a pair of boxing gloves and pummel the opponent senseless.
If this works out, one could see all three starters representing the
sky (Rowlet), the
land (Litten), and the
sea (Popplio), which is pretty ironic, seeing as the legends of Hoenn (another tropical region) represent the same domains of sky, land and sea. Ultimately, I could be wrong with my theory depending what the second evolutions will show, because the second evolutions give a better insight into their actual typing. Who knows, perhaps both Litten and Popplio will stay mono-types.