Disclaimer: I have no idea what system currently determines mob levels on Herocraft.
So far I have seen several comments relating to mobs, dark rooms, and how to motivate players to MOVE around the map. I propose a simple system to determine mob level based on location and time, instead of purely RNG. Mobs spawning in certain areas may be a much higher level than in other areas, and this changes over time. This would be implemented using Perlin Noise (If you are new to this concept, it is essentially a noise generator that functions in multiple dimensions; Minecraft uses it for terrain). You can understand it as a continuous function that can take multiple values and return a single value. Because the values would be location x, z, and current time, the areas of the map currently spawning the highest level mobs would change over time and never drastically change with any of the 3 input variables.
This would discourage dark rooms and farming mobs in a central location. Instead, players would venture out to farm the highest level mobs for the highest experience gains. If you think the extra experience does not outweigh the amount of mobs farmed in a dark room or the extra risk taken to travel to areas of high interest where you might have to PvP, that is because I haven't mentioned custom items yet.
The creation of a new item system is in overdrive currently and I believe it will revitalize the server. These items are important in many ways and tie into Heroes attributes, skills, and more. Obviously, the rarest items only drop from higher level mobs. The high demand for acquiring these items would tip the scales in favor of roaming the map to farm mobs. This in turn would cause a new interest in PvP, make players more familiar with the map, and make everyone feel much closer together instead of isolated in their towns.
Edit: There would be ways to know what areas of the map are spawning the highest levels of mob at any time
Edit: This also ties into the server economy because of level requirements on items. Players won't just take their hoards of money and buy the most OP item on the market, they will be constantly looking for the best item they can use throughout leveling. All levels of all types of items will be in demand and probably single-handedly increase market activity and item trades.
Moar Edit: Cool things like having the occasional very high level mob spawn around the map announced to the server with the absolute highest level loot would also be fun
Lastly: Whoever had the FANTASTIC idea of placing tons of "resources" around the map that just have to be actively maintained for your town to gain resources is amazing. As a long term goal for towns, I think generic "resource" requirements not tied to specific Minecraft items or amounts is a fun system that can foster community, variety in classes, and more unique game-play, while avoiding the stress of having to meet tax quotas, etc.