The basic things that cause RPG addiction are : Loot, Monsters and Skills.
Ideas for future moddables
-Item and Monster Generators, and Thing of the week or month
Pretty simple. People make monsters and items. Every week or month, voters or a Stardock employee incorporates one new Item or Monster a month. This can even be a spell, hut or quest.
What's the point of having mod tools if you can't extend the game for everyone?
Ideas for future
-Quests that grant you awesome loot or skills. The hero that completes the quests gains a special ability, item or trait increase. Currently the quest rewards are kind of lame. (Gold? Slightly more exp? BORING)
-Quests that are kind of mean: Maybe you unleash zombie apocalypse, or cause a volcano to explode in your neighbor's territory.
-Monster factions. Perfect for monster lairs. The cities could have fat loots, and the sovereign could be a dragon. The faction might guard an awesome resource node. Like 3 Arcane Libraries. Or 5 Fire shards. You could also negotiate with these factions. "Hey king dragon, here's 5000 gold, attack my enemy!"
-More awesome capstone techs. Diplomacy tree already has awesome capstone techs, Dragon and demon recruitment. Awesome. These capstones shouldn't just be "win the game". They should also be "Win awesomely"
The Civilization capstone might be something like: Magic Mill, convert resources into Mana. Stuff like that.
-Please, please put an end to building spam. THere's no reason not to just spam libraries. You can build 10 outposts spamming libraries like no tomorrow. That should be horribly inefficient. Instead, every city level should have awesome buildings. You might have a level 2 Library that grants +10 Tech, a level 3 library that grants +20, etc, etc.