As many of you may know, the final evolutions of a generation's starters tend to have some kind of underlying theme to help tie them together more. I've recently been looking into a few possible themes for the Alola starters:
Theory 1: RPG Classes Continued
This is probably the most common, widespread theory: The Alola starters simply continue the Kalo's starters' theme of RPG classes, which would be fairly self-explanatory given their designs and abilities.
Theory 2: Greek Zodiac
Litten's evo is the most iffy in this one. There wasn't really any other symbol that matched up with it as well, but lions and tigers are both big cats and the symbol also resembles a bomb, which would match up with Litten's subtle bomb motif. The greek zodiac is also of importance when it comes to alchemy.
Disclaimer: at this point, I'm probably reading too much into it. Credit again to Alchemylab.com for the descriptions