I hear a lot of people complaining about "we want more content" and then suggesting things that honestly wouldn't help in the long run.
A frequently suggested idea is Tier 4, in all its forms. But what would this create? All it would do is cause the players to get to the end of tier 4 and grind that, and then whine that they need a tier 5, and so on and so forth.
Another idea is more levels. But this, like Tier 4, is just the same old stuff, put into a new container in a different configuration. It doesn't alleviate the lack of "playable content" that everyone wants.
More weapons? Yeah, I guess, but you're still using them in the same old way. (and it only affects those who chose to use the new weapons)
Once the players have reached "end game", what more is there to do?
What would truly help, not only tier 3 players, but also tier 2 and 1s, is more enemies.
Not just one or two, but a plethora. I'm talking 15-30. We need jellies that split apart into smaller ones when attacked. We need brutish werewolvers that gallop around at great speeds, wall-running, throwing their weight around and in general trying to destroy the player. We need construct catapults that fire mini gremlins at the player, which then parachute down and just try to be a nuisance in general. We need tiny repair robots that buzz around on the ground which can heal or throw up a protective forcefield over its fellow monsters. We need jellies that pretend to be a treasure box, or perhaps even a fellow knight, and tries to body slam into the player when they come near. We need immobile plants that hide in grass or bushes and snaps at the player or spits long, spiny vines. We need giant rolling balls so we can Indiana Jones our way down long hallways. We need skeletal liches which cast curse over an area of effect. We need fire/shock/poison/ice-breathing drakes with limited flight shooting various projectiles at the player. We need more enemies that cooperate with eachother to create more deadly combinations.
This is supposed to be a planet constructed of various worlds. The lack of diversity in monsters doesn't really reflect that.
With more enemies, (not necessarily all in the one level, spread them out a bit) more strategies arise. More level configurations can be made, and the diversity of level building increases. More traps help too: the fire launchers in the Razwog level were a very good example. With levels feeling less stale, and more enemies arising that the player needs to learn how to fight, the game becomes fresh again. Certain enemies might be harder to fight, and can spawn more frequently on higher difficulties. You can play through a level you've done before and not feel like you've done it before.
This is probably a bad example but, of all my experience on the forums, the least complaints arise when new enemies are added.
Quite honestly, there aren't that many "roles" for monsters to be in at this time, with the Battle Arenas giving a general sense of the limited variety available. While the game is certainly enjoyable with the number of enemies currently existing, having a very large and thus more random selection would improve things.
For example, Wolver Dens are incredibly scripted levels, with only a few monsters besides Wolvers (in easily spotted rooms) and a preset arena. What if once in a while we expected to find an Alpha Wolver and instead got the unpleasant surprise of the angelic-but-deadly Skolver? (I'm convinced it exists. Somewhere.) Or Gremlin Hunters, purpose more or less self-explanatory?
Also, props to all of Hex's ideas. Just a note, though: you didn't suggest a new fiend monster.