Steam describes the crucible as a "level editor" but it seems its more of an arena-maker, linking arena rooms together and stuffing it full of mobs.

this is a serious disappointment.

Will we ever see any DLC to expand this to a true level editor where we cn link together actual dungeon rooms (and not sprawling arena areas) to create a single dungeon, add chests, lock doors, hide keys, specify loot drops, and add tricks, traps, and interesting features / story?

No, it doesn't have to be a full-on modding suite but some greater drag and drop ability to craft interesting dungeons crawls would be awesome. Link levels together to make whole adventures. Share them online. Keep the content going, keep the gameplay going.

Future DLCs could include more dungeon rooms, exterior areas, more features, loot etc.

Yes, i know there are no current DLC plans. if that changes, this seems a natural evolution.

Consider it, please, devs.

IMHO better a clever, well-implemented Dungeoneditor for every player and platform than a hard to handle and install Mapeditor that only PC players can actually use.