skins that give in game bonus stats

Technically, there already are. Any skin that's wearing sunglasses takes 1 less damage from Leona's passive. This is the only one I know of that is skin based, but there are other hidden things like this that are champion dependant.

Though it would be interesting to see something like Pool Party skins having a higher mana "pool" (preferably at the cost of something else), this doesn't set a good precedent and could dip the game into the pay-to-win category.

Thus is pay-to-win, so no.