There’s two versions of this suggestion
The Easy: Towers But Underground
Yup, that’s it
To add more variety, because it would be cooler to fight your way DOWN to a boss rather than UP to a boss
Visual differences
Well the atmosphere would be cool and stuff. There would be a small building above the dungeon to show that it’s there, similar in size to current in-progress towers. Smoke would come out to show if it’s been cleared.
The Crazy: Endless Dungeon Combat
Spawning
2-3 dungeons would spawn around individual seas in spots replacing enemy towers (probably add a few enemy tower spawns to compensate)
Aboveground
There would be a small building to indicate the existence of a spawned dungeon. Gather up to 3 other dungeoneers (number doesn’t really matter ngl) to start a dungeon party. Once prepared, you start fighting. Multiple parties could fight the dungeon simultaneously, and have separate cooldowns after winning
How it would be endless
Characters could be teleported below the sea floor, where randomly generated rooms are continuously loaded/deleted after eachother (or you just run in a complicated loop for better performance).
Rewards, scaling
Every 5 rooms (depending on size, concentration of enemies etc) a small loot stash will spawn, for example 1 rare chest and 2-3 uncommon chests. Every 10 rooms will have a miniboss with slightly better loot, similar to current tower non-main bosses. Then every 30 rooms will have a proper boss fight with more loot. Fight until you wipe or everyone wants to leave, probably 1 level per room that reduces the further you go.
There could be healing rooms to compensate for the continuous combat