I don't want to go quite so far as to actually pull out a pre-3e ruleset — there's a reason I preferred to play Tunnels and Trolls before the year 2000 — but what I'm getting at is something a little more fun and freeform -- "a sandbox game" to use the current parlance. The giants campaign, while it certainly has a lot of action, is still Teh Epic Storyline with pieces that need to fit together and moving parts I need to keep track of.
When I picked up Thunderspire Labyrinth, one of the ideas I was noodling around with was a 4E "pure dungeon crawl" side-game. In the end I discarded the idea based mostly on our group experiences with 4E combined with various "Now that we've been playing it a while..." posts I've read around the gaming blogosphere. But game system aside, the core nugget of a sandboxey game still appeals to me, and I might just go with it. A city, a tavern, a handful of 1-3, 2-5, and 3-6 dungeons in the neighborhood, and let the players get themselves into trouble. Sounds fun. :)
 The giants are mountain an invasion and they're just the tip of the iceberg, it's up to the PCs to find the cause and save the world. Yes, the idea's been done, but that doesn't mean it can't be done again!
 Hmm. This was a typo ... should be "mounting an invasion" of course ... but I kinda like "mountain an invasion" in reference to giants, too.