EDIT: Sorry for wall of text. Everything below may not be what you were after, but to me this is what i think about when i hear monster level in Tibia and the problems it could bring.
//EDIT.
Imo a monster leveling system makes no sence if player max lvl is infinite. If you put a max level on player, it should be easier to make sure that a monsters level and stats stay on a line above the players powers all the time. (not thinking of mlvl, skills, only lvl)
Buuuuut, if such a system would take place, i'd like to be able to double-click the monster in map-editor and give it a level there or even a level on the spawner and all creatures inside the spawner will get the level of the spawner. if you understand what i mean.
I'd like to be able to make a rookgard or a low level hunting area that contain demon creatures, but ofc as low lvls so you can do something more exciting than killing bugs and wolves.
A game like World of Warcraft has done their monsters in such a way that they can be used over and over again with different levels, and it makes sence. Hoggers, Any kind of Humanoid, Dragon Hatchlings, Animals, Imps etc.
But they also have the power to increase or make a creature smaller if they want, without the need for a new sprite. Like any bosses, they are most likely double the size of a regular monster in that dungeon.
Tibia basically have monsters in families, where most monster doesnt have a family (im thinking Pokemon now)
Rat -> Cave Rat.
Larva -> Scarab -> Ancient Scarab
Rotworm -> Carrion Worm
Wolf -> Warwolf
orcs ->
minotaurs ->
Dragon Hatcling -> Dragon
Monks
Bug
If you want a level system for monsters in Tibia, you have to decide which level a normal of the strongest(Demon) and the weakest(rat, snake) would be as a default. If they are higher or lower level than default their stats should follow a function based on their level.
Say that a Demon is level 100 by default and if you make a Demon(20), all of its stats, damage, health, exp, anything really to be reduced by X%.
If you want to make it even more custom, there should be brackets of loot tables that will be used depending on which level you put the creature. A level 10 demon shouldnt for example drop MPA.
In world of Warcraft if you kill a dragon in one area, it doenst have the same drops as a dragon in a different area. If you are after a recipe of some sorts, you go to the area of which it drops.
Tibia creature loots could be changed so that items below plate set is common, and above is crown set is epic and between is rare and all items that can be dropped from any creatures in the game (not quest items) will put into level brackets. The higher the level of the monster you kill, the higher the chance of a common, rare or epic item to drop.
This could also balance the profit/waste ratio of all hunting areas and make those special drop moments really exciting!, imagine being level 10 and you kill a creature level 12 and it drops a knight armor (which almost never happends)!