I actually really enjoy this question, because it's one of those things that you can let your mind soar with if you're a roleplayer. My main (and my guild) are very anti-Scourge and pro-reclaiming (and colonizing) Northrend, so Wrath is a fun time, and perfect for envisioning my guild in that sort of position. My character would probably do something dungeon-related, maybe in Icecrown, where the Scourge population is thickest. I could definitely see an escort-style quest, where my usual 5-man group in NPC form fought alongside a 5-man group of players in a Zul'farrak style encounter, fighting off waves of undead. After that encounter, those NPCs would 'hold off' more Scourge while the main party forges ahead to take out the boss and save the day.