I have a few ideas regarding mob mods:
1. Very Hostile Mobs
This mod would consist of very dangerous mobs. Mobs that have a high level of defense or a very high HP. Mobs that bust down doors, kill other mobs, and can break blocks that can be broken by hand or weak tools, and set things on fire to get to you and kill you. These mobs would appear above ground or below ground. Although they could break any block breakable by hand and certain middle of the road blocks (cobble, cactus, etc.) they could not destroy stronger blocks (metals, mese, diamond, stone brick, etc.)or chests. Some of them could also carry wooden wepons/tools. They also can not climb ladders. These guys will avoid light (although not take damage unless in sunlight) unless they are trying to attack you.
-----------------------------------------------------------------------------------------------------------------------------------------------------------------
2. Bosses
These guys would spawn ONLY in their tower(s) and their towers (tall towers made of wood, cobble, and mossy cobble). Their would be a variety of bosses in their own unique towers. Their towers (depending on the boss) would spawn randomly (but sort of rarely) on a specific type of block (similar to the Prymide Mod). Once you can find their tower you can enter and start climbing up it and fighting smaller mobs on your way. Once you get to the top you can enter the Boss's room and he will lock you in to the completely inescapable area with an inactive portal and a chest. Once you defeat him the chest will become accessible and full of good items. Once you empty the chest the portal will become active and allow you to transport right next to where the tower was. Also when you teleport the tower will instantly disappear.
------------------------------------------------------------------------------------------------------------------------------------------------------------------
3. Ore Mobs
These guys would spawn on the ore they are made of (a steel monster on a steel ore, a mese monster on a mese ore, etc.). These guys would need a weapon of their material to be defeated. They would also have a very large amount of hp.
------------------------------------------------------------------------------------------------------------------------------------------------------------------
4. Store Clerk
This guy would buy and sell all nodes, tools, ores, and tools found in vanilla minetest. His store would be found in villages. He would probable give you a currency for giving him supplies, which in turn you could use to buy supplies.
-----------------------------------------------------------------------------------------------------------------------------------------------------------------
5. Villages
These would randomly spawn on flat areas of dirt. They would be full of villagers who fight hostile mobs. The store clerk would be found in here. This village would be made of houses (some occupied by villagers who will attack you if you attempt to modify their house) other that are empty and open for you to build in. Villagers would either have unlimited HP or respawn very fast. They would also have some guarding the gate of the village. Most of the village would be covered with a wall, however there would be a big entrance to the village in the front with nothing but guards. Villagers will NOT attack you unless you attack them or their houses/village. Villagers would also repair damage done to walls and their houses done by players or mobs.