Whether it be meta tools or apps, or literal concepts like a mineral or a race, for example, a must for me is for the world to be built in a believable manner, and at the most simple, following some decent climate rules without much restraint.
my favourite would be drip plume ice worlds with hotspots that allow for rainforests in 80 Celsius steam zones, very exotic, the temperature regulated by these hotspots.
I rambled on too much…
Tunnels! A tavern that was swallowed by the earth but they just tunneled down to it and now its a gimmick. Ancient ruins that intersect with the surface. A shortcut between two buildings. An old mine that could serve as a base of operations. A cult’s secret underground lair. A gateway into a splinter dimension that effectively serves as a non euchlidian tunnel. A bit of the road that collapsed into a mysterious freshly-dug tunnel, taking a merchant’s cargo with it.