Disregarding the silouhette and what it merely looks like, I think, from a logical standpoint, that the troop is most likely to be either golbin, giant, or minion-based. Each argument explains and addresses points that the other two do not.
1. Golbins are one of the core troops of the game and the only "tier-one troop" with no hero. A goblin hero is also the most likely from a plot perspective; the "Goblin Lord" figure was the primary antagonist during the tutorial, and singleplayer takes up half the attack screen. However, this figure was never mentioned again, and any plot was discontinued after the tutorial. It would be advantageous for Supercell if they were reintroducing the plot (which I think would be really cool; I would enjoy cool fantasy storylines as much as the gameplay) by using this hero as a starting point. As for its abilities, I think it will more likely to be spell based (due to the staff) and can thus play a support role.
2. Giants are commonly thought of as support units. Maybe Supercell changed Giants with Goblins in the barracks screen in a previous update because they planned for Giants to be the next hero. After all, the research order for lower level players has not been changed.
3. Minions are the first of the dark troops, which are not represented with any hero. This also explains why the hero costs elixir instead of dark elixir, thus retaining the resources' complementary nature. It would also be the first flying hero, which changes traditional things up a bit AND matches the new defense's falcon appearance.
I'm not saying other possibilities do not exist. Heck, wizards, healers, witches, new eagles or falcons, or even sweeper heroes (LOL) all have their merit as potential canidates.
Really hope Supercell starts releasing sneak peaks...
