According to Blizzard Gnolls are quite dumb creatures, I don't think they could have a tech tree or organise armies like other races in the universe of Warcraft...
But they would be perfect for a mini-game map, or maybe a funny parody of a Blizzard's campaign: you could make the adventures of Paulsgnarf before he met Sylvanas, showing how weak creeps react to the facts told in TFT, with missions like "steal a leg from Kel'Tuzhad to show to the scourge the might of the gnolls", or "pretend to be dwarves and steal Lord Garithos' food supplies" (this would explain his hate against the other races in the Alliance)...
Or any stealth mission, without buildings, during a great event of warcraft ("Get the empty soul gem after Thrall used it")
Or you can tell the story of Timmy! Arthas rescued him from gnolls, but then a gnoll commando kidnaps him again, and after some events involving Kel'Tuzhad Timmy becomes a ghoul (as seen in the human campaign)...
then you'll have to explain How he became a penguin in TFT (as seen in the undead campaign)!