The trolls that scare other trolls
The dark trolls fascinate me for many reasons. First, they're the most reclusive clan of trolls and the ones who have remained isolated the longest. This gives them the potential to hold many ancient secrets and ways of life long lost to the other trolls. Somewhere in between the trolls and their loa and the night elves and their ancients stand the dark trolls, at home in the shadows, with magics no one has seen in thousands of years. That same isolation from each other has made it possible for groups like the Twilight's Hammer to pick them off, however. If the dark trolls are to survive, it may be time for them to stride out into the light.
As a player race, the dark trolls could join either faction. They allied with both during the original battle atop Mount Hyjal, after all. As a trollish race with no enmity towards the Darkspears, they could join the Horde. As the predecessors of the night elves and as former allies during that battle, they could join the Alliance, especially if Elune or one of the night elves' patron Ancients spoke for them.
Another reason the dark trolls interest me is that they're distinct from other trolls. They're about 10 feet tall, they have purple skin and prefer to live underground, and their society is said to be even more brutal than that of the jungle trolls. In the past, groups like the Zandalari have sought to unify the trolls, but even they haven't reached out to the dark trolls. The Shadowtooth Clan of the dark trolls helped save the world entire, and yet no one seems to want these guys around. Why? I have no idea -- but I'd love to find out.
Dwellers in the deepest darkness
One of the things I loved in both Wrath of the Lich King and Cataclysm were the underground areas. Azjol-Nerub and Ahn'kahet were beautiful, and Deepholm delivered a large, cavernous zone that felt like a massive underground world (even if technically, it wasn't). The dark trolls could easily have retreated to a subterranean redoubt to escape and in so doing, decided to come together and reject their isolationist ways.
Now, we could just go the usual path of having adventurers come down and wipe them out. But what if instead we come down and help out? If the Shadowtooth Clan has violent, expansionist rivals who want to lead the dark trolls to a war against the surface, it would be prudent to step in and help our old allies. (Remember, they count as allies to both factions, as they haven't been around to see the Horde/Alliance war heat up.) Perhaps in so doing, we'd win the dark trolls over to either side.
This would be a chance to really explore an underground world alongside a completely new kind of trollish culture. Are there nerubians down there? Other wandering earth elementals, spawn of the Old Gods like the forgotten ones, entirely new kinds of horrors? Long-lost colonies of earthen, mechagnomes, iron vrykul and obsidian destroyer tol'vir? It's a vast realm below our feet, and the dark trolls would make an excellent group to lead us down there.