Suggestion: Land of CarnageWhat is Land of Carnage (or LoC)?LoC is a mode unlocked after collecting x rare items (tickets, maps, or some kind of substitute) in Item World. While in LoC mode the enemies are boosted (in short difficulty mod is squared and easy/normal are no longer accessible). Also the game's strongest/secret ring of blood boss, {insert new boss here} can only be fought in LoC mode.
Incidentally Land of Carnage can only be entered for a limited amount of time. (Or even better: until defeat! Which would spice things up.) The tickets (or substitute items) are consumed upon entering LoC and can only be found in normal HV mode to prevent perma-staying in LoC.
As for effects, simply put the difficulty mod is squared, which means so are:
EXP Modifier, Credits Modifier, Monster HP Modifier, Crystal & Monster Damage Modifier.
(Which also affects damage done by monsters, amount and chances of equipment level gains in the Item World and drop quality.)
There's also another visible change to go along with this mode. There's a color swap to clearly distinguish this new mode from normal HV mode.

(In this simple example I swapped the background colors and text colors in the css. This is just to make a point that it's easy to code.. It should definitely look better than THIS.)
It'd also salvage Item World that everybody hates so much.
/you're welcome (IMG:[
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style_emoticons/default/smile.gif)