The usage of Arch-Viles as boss/guard monsters should have a limit...
But you should also be careful not to tone it back too far, as some of the crazy 'unfair' challenges can be the highlights of a wad, forcing one to adapt and perhaps use some novel tactics.
This also gave me an idea for an additional option for monsters; 'difficulty spike', ranging in strength (rather than frequency), which is how high above the normal difficulty the game can go, and effects both the strength and number of enemies.
Most of the great games I can remember have some pretty big spikes in difficulty, and whether by accident or design, it creates memorable moments and exhilarating victories. Also I desire being able to make relatively quiet maps with some huge spikes in difficulty.
Whilst it may seem like an unimportant thing to add, I think it is something that makes sense to be user controlled.
Also been thinking about trap usage. I'm not quite sure how the difficulty of a trap is determined, but here's my thoughts on it:
First determine the priority of the target of the trap.
Top priority targets are keys, and anything that is essentially unavoidable in order to finish the map. Although oblige does not do them, a trap that spanned a corridor that must be passed would also be top priority.
High priority items are new weapons, soul and mega spheres and blue armour, maybe also large plasma packs.
Low priority items are weapons you already have, green armour, 25 health pack, other large ammo packs etc. Also an avoidable trap with no item associated with it would be low priority.
Low priority targets should only have small traps associated with them (couple of guys on early maps, couple of revenants on later ones.)
Also you should break even or make a net gain on ammo pickups. For example collecting a shotgun for its ammo, only to spawn 10 imps is a bit cruel. (not that this very likely happens in oblige at the moment)
Higher priority targets can have tougher traps associated with them, but don't have to, its fine to have a small trap, though it should probably tend towards difficulty especially on later maps.
Dead ends that don't have to be revisited can have huge, over the top traps, that are better run from, though in this case the enemies should always spawn or be released in the same room as the trap. Also the priority is possibly less important in this case. (This should probably be optional as it may frustrated completionists).
Also because running for traps is sometimes a legitimate option, ammo (and health) for closet monsters should tend to be in closets for that trap, so if you avoid the fight, you don't make a net ammo profit.