[Zanny]

Areas are vast, a dynamic spawn system would not necessarilly increase the density of mobs, just the frequency of their respawning or the distance between spawn points.

I wanna assure you its the PCs that will tax the PC more than anything with input and animation functions. NPCs should be easier to render and recognize, especially with duplicates, and having more mobs doesnt mean more people, it just means people finish their quest / search for an item quicker and leave sooner.

Quickly wanna throw a token loot system back into the fray -

When you kill a boss, it drops a token exchangable with an element in the enviorment that can grant you ANY item on that bosses "loot table" (a 24 man raid boss would drop 12 items, for example, 3 tokens would drop and you could use them to obtain 1 from groups of 4 items from his loot table) and the boss, as well as variable trash mobs and some difficult small group content instance mobs, could drop a cumulative token (or reverence score, a fill-able bar?) which counts up from 0, when you get high enough you can exchange "reverence" or a stack of said tokens from difficult encounters (maybe taking a castle in the abyss could also award one or two) you will be granted a powerful item from your faction (aka, faction specific gear maybe).

Thats a little in depth, but you get the idea again.