Most cat Pokemon usually evolve into a bigger cat with very little change in design so that's what I fear for Litten's evolution but knowing Game Freak they'll probably make it more tiger-like (heck, Chinese Zodiac pattern anyone? Litten is the tiger, duh) and there might be another unexpected thing they'll do here because starters take a lot of time to design so they have to make them unique in a way.
fire type starters always turn into something know for being a huge force.
GEN 1: charmander into charizard, one of the first dragon pokemon(even though it didn't get the type til 6th gen via mega stone)
GEN 2: cyndaquil into typhlosion, which is pretty much a wolverine; an animal known for having strength & ferocity out of per-portion
GEN 3: torchic into blaziken, or as i like to call it megaultrachicken. it seems to be a hybrid of the sun god Ra, a phionex & a tengu.
GEN 4: chimchar into infernape which is most likely based on son goku, the monkey king who was sealed by the gods after going out of control in journey to the west
GEN 5: tepig which turns into emboar, which is based on a
military commander from Romance of the Three Kingdoms.
GEN 6: fenniken into delfox, which seems to be a mix of a fox, an oracle of delphi & a witch or magician with a bit of hawaiian fire dancer
so what ever it evolves into will be something known for being a force to be reckoned with.
also their not gonna go making the starter look too much like the mascot unless it was the mascot, which hasn't been done outside of gen 1 games & remakes of those games, or has some relation to the mascot which hasn't been done before
Solgaleo is based on the alchemical symbol of the green lion devouring the sun, relating to the notion of purifying metals and looks a bit like liger zero from zoids, only more cosmic & lion-like
this brings us back to the 4 guardian thing & the starters +1 hidden secret and my theories on it