For final starter types, if the 20th aniversary is any indication (or maybe its not), I'd predict the following type combos for the Alola Starter Evos:
Litten's Evo - Fire/Dark or Fire/Ground:
Fire/Dark is a possibility and could be pretty cool in general as a starter type, especially if that Heel theme ends up true. For
Fire/Ground, Litten's evo would be the prime opposite of Charizard representing the Sky with its secondary Flying type, with Litten representing the Earth. This in turn is reflected in a sub category related to the Chinese Zodiac, in which the Tiger and Dragon are considered to be
eternal rivals, representing
Earth and
Sky respectively. More, regardless of Litten's possible secondary typing could be, either speculated type would reflect well on the 20th aniversary of the franchise. After all, Charmander started off as a mono Fire-type, but became part Flying when it evolved into Charizard. I also expect that Litten's intermediate evo stage will stay mono-type, as to reflect on Chamander's first evo stage that's Charmeleon which was also mono Fire-type.
Rowlet's Evo - Grass/Flying:
Not much to say about this part, other than the idea that Rowlet represents a Grass Starter that starts out dual-typed right from the start, and the only other Grass Starter to have a dual-type was Bulbasaur, which started out with, and stayed with the
Grass/Poison typing. If true, this would go nicely with the 20th aniversary of the very games that jumpstarted the Pokémon Franchise into a worldwide phenomenon its today.
Popplio's Evo - Water, Water/Fairy, or Water/Ice:
With the reveal that Poppilio can learn Disarming Voice, a Fairy-type move, there is a fair chance Popplio could end up becoming a
Water/Fairy type, especially if that siren theme is true. However, its also fair to expect the possibility for Popplio to stay Water type throughout its entire line, especially if the Alola starters are designed as such to reflect on the Kanto Starters, of which Squirtle was the only Kanto Starter to stay Water-type through out its evolutionary family line.
Water/Ice is also a possibility, but frankly, I don't expect it, as Water-types in general can just learn almost any Ice-type move without being Dual-typed. They just miss out on the Ice type STAB.
My final verdict:
With Sun and Moon being the 20th aniversary games, I speculate/theorize that the Alola Starters were designed to follow a reflecting theme, as in reflecting the way of how the Kanto Starters got assigned their types.