The current categories are, frankly, underwhelming - no offense to the creators or runners, but I think we can agree on that. They are, excepting Goblin Siege, a short, simple race towards an immediately achievable goal. My idea is kind of self-explanatory - become a Mountainhome as quickly as possible. This is a solid, non-arbitrary goal (it's an explicit goal in-game, with its own screen to track progress and everything) that would require the runner to actually engage with the game mechanics on a somewhat meaningful level and (theoretically) create an enjoyable run with more depth and nuance.

Your architecture, roads, and offerings must each reach a certain respective wealth value for the Monarch to migrate to your fortress, therein making it a Mountainhome; the offerings are specifically notable because the caravan must safely leave to deliver the offerings, adding another layer of complexity. This involves significant interaction with the game's mechanics, while (unlike other categories) having lots of room for breaking those mechanics apart. Routing, strategy, optimization - yknow, speedrunning bullshit. Most importantly, the ultimate goal is just far enough away to allow the runners some creativity.

In a game specifically without built in goals, I feel this is one of the few, perhaps only, goals for a run that isn't arbitrary, is supported in the game, and could result in a category that's actually pretty enjoyable to run and watch. DF is about to enter a golden age with the Steam edition, so a category with some appeal to runners and viewers would be huge.

Obviously, I can't say for sure how good the run would be until people are actually trying it out. But DF needs new categories bad, and after thinking on it for a while it's the best run I could think of.

One note, the Monarch will alternatively just come once you have adamantine, so I think if there were a category like this, it would be served well by being sorted into Mountainhome% - Adamantine and Mountainhome% - No Adamantine, or somesuch.