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The evolution of the frayed hair of Litten's tail to more a club shaped tail is pretty convincing now that we have the middle evo. One of the things it doesn't have going for it is the big orange cowlick going up and away from its head that is nowhere on the final design.
Rowlet's evo now has longer legs, so it makes sense that they get even longer in the final. Also the seeds under its wings correlate to the final's little ammo magazine under it's wings.
Popplio's evo is the most convincing as the skin gets lighter as it evolves and it's ears turn into hair as well as gaining a ruffled dress that will flatten out and extend to the end of its tail for the final evo.
The only thing that bothers me is the coloring of each of the final evolutions, but this could just be the primary designs and these may not be the final colors.