In every game, usually speedruns are about killing/destroying. But Terraria is also about building. So I though, building something would be a refreshing and interesting type of category.
The closest I can recall is house% in Minecraft, but I think there are a lot more interesting things.
The hard part is finding and standardising buildings for runs.
Some of my ideas;
pyramid or some kind of cultist building
house in every biome%
castle/mansion with specific furniture,etc
functioning real life house(electricity, bathrooms,etc)
So the runs would include collecting specific blocks, the building itself and usually some bossfights.