@.access:
Popplio will probably get some sweet coverage with Acrobatics and hopefully this will remove the need to give it Ice Beam (I don't want it to become a better version of Walrein).
I don't know what its second could be, but I'm in for anything as long as it doesn't remain just Water. We already have too many pure Water starters.
Litten could be just Fire and I would be ok with it, Typhlosion is the only pure Fire among all the starters so it's fine. It is a terrible type in the defensive aspect, but I am pretty sure Litten will focus on offensive stats.
Rowlet… as much as I want it to lose the flying type, the other options people are offering are not much better.
Give it long secretary bird legs and I can see it becoming Grass/Fighting (the Fire/Fighting days are over because the Grass/Fighting days are here?), but Grass/Fighting doesn't look much better than Grass/Flying, except for resisting Stealth Rock.
They could go with the whole owl characteristics and make it Grass/Dark. Again, lots of weaknesses and resistances, but getting 4x damage against U-Turn is never a good thing. Also, I'm afraid it would become too ninja-like and look like a Greninja wannabe.
Out of the options Grass/Steel is the only one that seems better than Grass/Flying (they could for the knife throwing aspect):
! Immune against Poison
1/4 damage against Grass
1/2 damage against Dragon, Electric, Fairy, Normal, Psychic, Rock, Steel and Water
2x damage against Fighting
4x damage against Fire
Funny that you mention it. Out of the many possible combinations with the grass type, flying, bug, dragon, fairy, dark, psychic, steel, fighting, hell, even ice, are 4x weak to at least one type. Besides, based on its constitution, the only types I could see Rowlet evolving to if it ditches the flying type would be either psychic or dark.
And despite this, grass/flying is the best possible combination, specially for a bird pokemon!
Birds are light, and since I see Rowlet staying grass/flying as it evolves, I have no doubt the cute critter will have a high speed stat.
But what people seem to forget, ans this is important if you plan to use Rowlet in competitive, is that there's a very useful flying type move which for only the turn is used disables the flying type of a pokemon and at the same time heals some hp: Roost.
By using that move Rowlet's weaknesses would change to:
! 1/2 damage against Grass, Electric, Water and Ground
2x damage against Fire, Bug, Flying, Poison and Ice
Pretty useful if you want to test out if a water pokemon knows some dangerous ice move, or if you are not sure you can finnish your opponent in one turn.
Alternatively you may use Protect and come out unscathed of your rival's pokemon next move.