A castle in the air of mine would be to produce a
Gorkamorka MMORPG. There was actually a
Gorkamorka racing game in development for the Dreamcast which was unfortunately canned, although development screenshots and an Alpha build are floating around the internet:

But I'm imagining something more expansive. There has been another vehicle-based post-apocalyptic MMORPG,
Auto Assault, and while that failed due to bad environmental design and a buggy launch, the core concept was sound and with some refinement it could be a dramatic and distinctive affair. Roaring across the sand-scoured, mutated landscape of Da Skid at the wheel of your Kustomised Rig, a gaggle of Boyz hanging off of guard rails and howling with glee as the wind rushes past them, skidding and sliding around rival mobz as you scrap over Skrap (damage bonuses rely on the speed you're travelling at, forcing you balance acceleration and avoiding ramming into obstacles?), and dragging your booty back to Mektown where you are able to actually see Da Hulk be reconstructed in real-time, ready to blast off in the expansion set. Diggaz, Rebel Grotz, Muties and Tinboyz can be the creeps for PvE combat.
There is currently a 40k MMO languishing in development, but I fear that it's trying to incorporate too broad a scope (as well as being handicapped by not being
World of Warcraft). The universe is too big for that, and a tighter focus is needed to be successful.