Broderbund had a great game in the 80's called "Lode Runner". One of the best features (IMO) was the ability to design your own "level" and save it ~ sharing it with friends, or running it on your own.

I think the ability to design your own WoW instance - or populate an existing instance - would be a lot of fun, if implemented correctly. You could select an instance (or build a random one from existing themes - underwater, night elf ruins, haunted castle, etc.), determine level range, mob table, boss table, and loot table from pre-selected options, and the system would populate it accordingly. So...

Hob's Dire Maul:
Level 15 - 18
Mobs: Shadow Raven Priests, diseased ghouls, felguards, floating eyes, polar bears (all level appropriate and randomly distributed between stationary and wandering).
Bosses (names chosen from random name-generator): Voidwalker boss / Dwarf paladin boss / Moth boss
Loot table: level-appropriate, emphasis on warlock gear.

Hob's Wailing Caverns:
Level 74 - 77
Mobs: Grimtotem tauren, lashers, ancients, bats
Bosses: four murloc bosses, which summon a final tauren boss
Loot table: level-appropriate, class emphasis exchanged for 0.1% chance to drop a random mount or random epic weapon

Just ideas, but you could definitely have a system that controls the mobs, loot tables, and bosses, without being a welfare instance or a d*** in a box instance.