I think this is a good idea, but could use something more in it. Robots spawn at a 'factory' block that is generated on top of exposed stone and gather resources in an adjacent safe. Approaching the factory or safe turns the robots hostile. Eventually, they'll have a decent sized quarry, but resources will be expended by them as they are collected, so if you want to steal them, you need to make a move sooner or later.
Needlessly complex stuff, but for the sake of depth, and hell, it's fun to type:
When a resource type is consumed in X quantity, it modifies their behavior.
- Coal makes them work and progress faster or more efficiently, may also increase endurance assuming they start building nano stuff.
- Copper(?).
- Iron increases their numbers or even adds a second type of bot.
- Tin makes them move faster because it's a lighter metal and building material(Albeit flammable ;)).
- Gold would expand their control area and hostility range or something.
- Redstone and Lapas +1 to tech tier(totals in +2 if/when they get both).
- Diamond makes them immune to fire/lava.
Allowing myself to stretch into the far fetched: They get obsidian and construct a nethermine that somehow collects materials from the nether. If you enter the same region in the nether you might notice a flying drone carrying blocks similar to endermen, but to a hub that ports it back. This gives access to glowstone which, well, that just makes them a bigger threat, and by this time, they've finished their development and switch to extermination mode.
Eventually, probably once they reach the copper phase, they start building defenses or barricading the factory and safe by building walls in a symmetrical pattern.
If you manage to break the factory you get a basic machine or a fresh advanced machine block. Looting the safe is its own reward, but never gives more than raw materials.
If any robots remain, they may rebuild the factory assuming they still have the resources.
Take or leave any of it, I'm not married to the ideas here, its your mod, and it would probably take a dedicated team to do this. But I think IC² could use something that actually makes all the fancy equipment worth it. As is, I do fine against the standard mobs already, so I don't see much benefit to spending resources on fancy weapons other than the chainsaw. And the same ol' same ol' just isn't cutting it for me anymore, as I'm sure Killerbeez agrees on that point.