I know water theme has been already suggested, but it's not about new stratum theme but rather new content for end-game players
It's only rough idea of course.

Basic concepts

General:
- Additional Gate/Gates would be available in Haven. That Gate would lead to underwater levels only.
- It would have the same number of Tiers and Levels as regular Gates. Every Tier would also have one of regular themes like Beast or Construct, but contrary to regular Gates it would have submerged levels.
- There will be new underwater, swimming monsters in every family, like Draconewts (Beasts) Jellyfishes (Slimes) submarine-like-robots with crab-like-claws (Constructs), Scuba Gremlins (Gremlins) and so on.
- Knights would swim (new animation). Levels would be made of pipework, so there would be no flat floors.
- Underwater darkness limits knights vision. Player would see environment and enemies only in close vicinity.

Equipment:
- Regular weapons won't work underwater - new weapons will have to be crafted/purchased before knight dwells into Submerged Gate.
- Shields/armours and helms would work in Submerged Gate just like in regular ones.
- There would be several underwater weapon lines for every weapon type: Spears-like swords, Harpoon-like handguns and Naval Mines.
- Additional drops, alongside hearts and coins, will appear throughout levels: Bubbles (see below).

Oxygen:
- Additional bar will show up when entering every level in Submerged Gate: Oxygen Supply.
- The bar will be gradually vanishing for entire level - if it drops to zero knight dies. Of course it is not a case in Terminals and Sub-towns.
- A knight can give half their oxygen to a dead party member (only if he dies because of lack of oxygen).
- Reviving with energy replenish entire Oxygen Supply (only if knights dies because of lack of oxygen).
- If player picks up Bubble its Oxygen Supply will be replenished by a given amount.

Summary
- Levels with time limit will introduce additional challenge for players who have mastered regular game play. Overwhelming time pressure should be most significant factor in submerged levels.
- Players who have mastered regular weapons will have a chance to play with new toys in new environment.
- Additional content would be introduced for players who have already acquired all desired 5 rank gear. Underwater weapons would consume a way more materials then regular ones (mat sink for end-game players).