In a strange (and mildly disturbing) sort of way, Treecko is kind of the most appropriate Pokemon for the main character to become. It was never fully explained why the PC transformed into a Pokemon, but given that it was a result of Darkrai attacking them and Grovyle during time travel, it could possibly be that the player's "essence" or "spirit" or whatever partially fused with Grovyle's, and that was responsible for their tranformation into a Treecko. It's not much, but in other cases you're left with the PC transforming because the plot needs them to transform (which logically speaking is probably the case, but I like to fill plot holes and Mcguffins with at least plausible explanations wherever possible).
Although it's my headcanon that the Voice of Life from Gates is the voice that speaks to you at the beginning of every PMD game (with the exception of PSMD, where it's later revealed that the voice at the beginning of the game is Mew's), and represents some sort of fate-like force that brings you to the Pokemon world and then guides your journey. For whatever reason they decide you're better off as a Pokemon than a human (maybe they decide that you'd be more capable of fighting the opponents you'll need to fight as a Pokemon, or maybe humans simply can't exist in the Pokemon world for extended periods of time, and transforming you into a Pokemon is a workaround to this), and Darkrai's attack on you and Grovyle in PMD2 was simply the most convenient opportunity to transform you. If the latter is the case, that also explains another mild plot hole that I noticed; if Darkrai's attack would have killed Grovyle, who can survive Flamethrowers and Draco Meteors and Rock Slides by getting knocked out at worst, surely it would have killed you, a squishy human being, and yet it didn't. Maybe the Voice of Life protected you from Darkrai's attack. The alternative is that you survived the attack by sheer willpower alone, which given that it's a Pokemon game isn't really out of the question either.
Anyways, kind of went off topic there, what are people's thoughts on the artstyle for these new games? It seems to be a blend of the new 3d graphics style that was present in Gates and PSMD and the old pseudo-hand-drawn style from the first two installments. There's a weird sort of papyrus-like quality to the coloring that I find particularly interesting, as it wasn't present in the original games, but was in many of the pieces of official artwork for them.