Adventure maps that would be generated using the Geode caves generation, but with a shadow realm influenced theme (as well as un-breakable walls, like shadow dungeons -- this is mostly to ensure integrity of "special" rooms). The general idea being that the shadow forces are invading geode, because they've discovered ancient fragments of the Moon Goddess that they're using to create new Daughters of the Moon. Using the geode caves generation would allow the creation of vast underground adventure worlds, which could include special rooms (similar to the vaults and points of interest in the geode cave exploration mode) with special encounters. Enemies could get tougher, and have higher darkness levels the deeper into the caves players venture.

Special room ideas:

Onslaught -- Starts a timed event that spawns several large waves of enemies. Defeat all enemies within the time limit to succeed (should scale number of enemies per wave based on number of players in room when activated).

Mini-titans -- variants of titans from the shadow tower (Prototype Spike Walker, Weeping Acolyte, Scion of the Pinata God, Hydra Eft, Darknik Juggernaut). It would be neat to have a special block type at the "airlock" for these rooms that blocked entrance/exit after the mini-titan had been activated, and only became passable once the mini-titan was dead or all players were dead.

Daughter of the Moon Ritual -- activating this room would spawn 2 to 8 "Moon Goddess Fragments" around the room based on the number of players in the room. These fragments would periodically spawn enemies while active, and could be deactivated by the "light" stat of nearby players (within 4 blocks) being higher than the darkness stat of the nearby enemies for a period of time (like a point capture event). Once the piece was deactivated, it would stop spawning monsters, but could be reactivated by enemies being near them for long enough (should attract idle enemies while not producing enemies of its own). If all players before deactivating all the fragments in room or time runs out on the event, should spawn a Daughter of the Moon enemy that persists until defeated. If completed, should spawn a friendly Aspect of the Sun Goddess NPC that could offer special quests for the area.

Avatar of Q'bthulu -- Perhaps if players venture deep enough they could stumble upon an ancient slumbering Avatar of Q'bthulu, seeded to geode unknown aoens ago. This could be a stationary titan (similar to hydra) that spawns tentacle enemies to damage and knock players, as well as eyes (god, so many eyes, everywhere!), and maybe some acid. Could maybe start as a giant mass of eyes and tentacles, and shrink down as it loses health. Should recover some of its health if a player dies in its room or maybe just cause them to burst into more tentacles and eyes.

Rewards from events could be things like topside caches, crystalline cores, or a secondary currency (perhaps limited to mini-titans/ritual and/or maximum daily amounts) that could maybe be used for things like empowered gem boxes or lustrous gem boxes.

Edit (5/20): Had some other ideas I thought I'd tack on regarding this.

Would be neat to keep the population on these maps low, using the max of 16 that's currently in place for Geode caves, or knocking it down to 8 for this area. Enemy darkness levels shoulds start around the same levels as U10 (enemies/chests in this area should also have a chance to drop Crystal 3 gear, like in U10). Venturing deeper into the caverns should uncover enemies with increasing darkness values capping out somewhere above the maximum light value, makinge groups more valueable for deeper excursions.

Include treasure rooms as potential "special" rooms, and allow for some ambush variants. Treasure rooms would be rooms that would spawn some number of chests that could be broken for free loot for players that happen upon them. For the ambush variants of these rooms, chests would instead spawn groups of enemies when broken. (and perhaps breaking one chest would cause them all to break and release all the enemies). Alternatively, the ambush room chests could be a new type of "mimic" enemy (like the winking tentabox ally) that activate if struck by a player.

Have an NPC at the map's spawn point (like the NPC used to adjust equipment/reliquaries/etc in the normal caves) that sells consumable buffs for the caves that grant things like extra light, attack speed, damage resistance, etc., but are removed upon leaving the ancient depths (only function in this map, and are removed if the player changes maps). These could be sold for crystal (maybe add crystal drops to enemies and/or crystal spawns here, like the normal caves) or for some secondary currency specific to this area.

For special items that might be neat to see in this area, rings that have a special proc. These could be something like "Infused Crystal Rings", and the procs could be focused around the light/darkness interactions. Some ideas:

Blinding light - Explode in brilliant light, stunning and damaging nearby enemies. Enemies that have darkness higher than your light are stunned for longer, enemies with less darkness than your light take more damage. (20 sec cooldown)

Dark Appetite - Critical hits against enemies restores some health. If that enemy's darkness is higher than your light, you also gain a stacking light bonus. (1 sec cooldown)

Sun Seal - Defeating an enemy causes it to explode. If the enemy had higher darkness than your light, nearby allies are also healed. If the enemy had less darkness than your light, also creates an firey area that damages enemies for 6 seconds (15 sec cooldown)

Luminary - Critical hits against enemies cause a stacking darkness debuff. If that enemy's darkness is less than your light, also grants you a stacking light buff. (3 sec cooldown)