There are some clues in the descriptions about how the starters will turn out to be, which I'm surprised no-one has mentioned yet.
Rowlet:
"drawing near to its opponent without being noticed, and then lashing out with powerful kicks"
So Rowlet is clearly a physical attacker, that much is obvious. The not-being-noticed thing kind of points to priority, which ties in with the gale wings idea someone had.
Litten:
"Logical but also passionate, Litten always remains coolheaded and doesn't show its emotions on the surface."
The most tenuous of the three, this suggests that Litten will be a special attacker, but doesn't really say anything else other than that. In general cat-themed mons tend to be frail and fast so there's that, but it's kinda stretching.
Popplio:
"Popplio's swimming speed is known to exceed 25 mph."
Ok this couldn't be any more obvious, clearly a mon based on speed. Interesting because the design would lead you to think of something slow and bulky ala Walrein so the final evolution will probably be much more sleek like a real sealion. Possibly a mixed sweeper in the same vein as Infernape and Greninja since neither attacking stats are obviously emphasised, or it could mean it ends up being more status-focused which could tie in with its water balloon attacks.
"Popplio can snort out balloons made of water! Watch it spin water balloons into a playful battle strategy!"
Anyway nothing concrete there just some food for speculation :]
Edit: from here
http://www.serebii.net/sunmoon/pokemon.shtml