I actually like the purple eyes on Unicron. It kind of fits with how Dark Energon is purple and Megatron's eyes turned purple after he infected himself with the stuff.
From an artistic perspective, I purple eyes could potentially look better on Unicron than green ones, if they decide to give Primus yellow eyes. (Given Primus makes any sort of appearance) For anyone familiar with a color wheel, blue and orange (the colors Primus and Unicron are primarily) oppose each other, as do purple and yellow. It would help to make a nice contrast between the two, I might be looking too far into it. For anyone wondering what I'm talking about, click here.
I'm also with everyone who thinks that the volcano erupting was just symbolic. The fact that there seems to be a planet behind the Earth in the trailer is ignored in comparison to it, though, and I honestly think that's the biggest hint that it's probably Unicron. It also kind of looks like Dark Energon is falling from the sky, which just makes me think it's probably Unicron, considering he's the source of it.
Also, since it says "HE IS COMING..." at the end of the trailer, I honestly think that's more fuel to the fire of this being Unicron. That makes it seem like they'd go with the most obvious identity for the character if you ask me, and when most of us look at this, we think "Holy scrap, Unicron!" as opposed to Alpha Trion, Sentinel Prime, Liege Maximo, etc.
It's a really neat looking trailer, and all of this new information is definitely getting me pumped for this series.