Invisible monsters playing hide and seek may sound funny, but why go hunting them if there are several leveling and farming grounds which give better benefits for far less work ?
You sort of have such an experience in ToF where those two groups of monsters teleport into the dungeon in front of and behind you.
This however might be an interesting idea for a new map or dungeon - maybe a mix of visible foes and "invisible" ambush groups - but this also will not solve any of the pressing issues.
Class balance in any aspect of the word. Halfbred skilltrees, skilleffects, persisting bugs since 2007 and the like, you have read and experienced it often enough if not too often.
A new kind of balancing problem has just been implemented, the so called elemental system which totally ignores the only class actually relying on elemental powers that fail it since 2007.
As far as already can be seen and expected, the "ex" classes are not meant to freshen up the game but to replace the old classes so every issue of the old classes will persist, plus the ones coming with the ex classes as the game is always retuned for the strongest toon.
Personal experience with staff differs greatly... Letting this aside the gaming experience is what will make players stay or leave. Now enter the game and look around with "newbie" eyes. Then remember what kind of still existing problems got reported since when and what was done about it. Done, not talked.
You might get an idea why LC might attract but not keep new players.
Keep on hoping, hope does not need facts.