Yes, I agree. Actually I think your idea of customizing each world created should be a expanded, and made a basic part of the game. Most things in the game should be adjustable: Density of resources, types, density and rate of, 'monsters', lairs, availability of some, if not all spells (and the mana cost to cast), [users.that hate the teleport spells can ‘un-tik / unselect them thereby making them unavailable in the game they are setting up.] unit types, equipment costs, equipment values (Armour rating, attack defense strength, MP), etc ...
I think you get the idea. Just about everything should be customizable by the user when they create a world. You want horses and wargs to have more MP’s, tic the box, customize you game.
Sliders (as suggested above) could be used for adjust the overall number of specific resource / roaming monsters / monsters that (mostly) hunt wooded and /or mountainous areas available. Especially things for which we really don’t have a ‘hard’ number.
Sliders could shift the number of resource tiles from “almost none” to max for game/whatever that is.
Each spell could have a tic box. Checking the box for that spell makes it available in the game. Following the spell would be a slider that allows user to modify the base mana cost to cast the spell (low med, high,) etc. Or a box where you type in a number (within a set of range) to fix the base mana cost of the spell, the AP, initiative, or whatever factors the game engine is using with that spell.
Units could have a set of check boxes after them. Checking the various boxes would modify that unit’s (in its basic from) str, def, attk, MP, sighting, etc.
Finally: There should be a way to “save” the customization choices under a name, so it can be used again, without going through all the selections.