while playing and exploring a bit i thought about the way some mods (or even vanilla minecraft) are forcing you to explore to get iinto high-end stuff like factorization with the logic matrix programmer or thaumcraft and its knowledge fragments. I like the idea of things you can't craft because they're to high-tech or from another world. So here is my Suggestion:
[Worldgen] Reactor Ruins with Loot
So What do you think about a rare worldgen (like vanilla temples) of ruins of a reactor which melted down years ago. Doesn't have to be much but a few reinforced stone and glass blocks, maybe some low-tier machines like iron furnaces and a couple chests with loot inside. And i thought about a rare item which you can just get through exploration to achieve the high tier stuff like matter fabricator (maybe an ingredient for advanced machine blocks!?).
There can be also some radioactive effects in the area around.
I hope you like the idea. If there was allready such an idea, my searches in this forum doesn't lead me to something similar.