Putting The Mine In Hack/Mine
It'd be nice if spelunking and mining were as interesting as PvE and dungeoneering are currently. For this I have a few suggestions.
1 ) Tougher mobs. Tougher than exist on the surface above the same area, anyway, with rewards to match. And more mobs in general would be nice as they're currently few and far between underground. And mobs unique to the underground setting would be niffty as well.
2 ) Random stuff!! The odd random lewt-filled chest or cluster of breakables deep in a cave somewhere would make the spelunking trip seem way more worth it than some measly diamonds.
3 ) Deep dungeons. Dungeons only accessible from caverns deep underground. The mini-map would totally make them easy to spot. Finding the entrance though might take some splorin'.
4 ) Y-axis specific ALVLs. Madness Chasm? Inferno Caverns? Terror Tunnels?
1 ) Secret doors and hidden dungeon rooms. I don't know if it'd be easier to do this once instanced dungeons are employed, but it'd be neat to have dungeon rooms hidden on the mini-map until they're entered. Then you'd be able to add hidden and secret doors and such. Which brings me to,
2 ) Blocks only destroyable by explosions. They could just be the cracked variety of the current dungeon blocks. But basically you could use them to wall off sections of dungeon. It'd make TNT and the ranger's lewt-obliterating explosion arrow that much more useful.
3 ) Split the inventory/stats/skills panels into three seperate tabs. Include things like total resistances and DPS calculations on the stats tab. With the extra space on the inventory tab you should be able to include a full inventory or at least pare it down to two inventory sub-tabs.
4 ) Different colored chests with different quality of loots. I always enjoy the thrill of seeing a fabled golden chest. Mad props if you can add huge +size golden boss chests.