This thread covers the basic thing that has bothered me about modded Minecraft too. I don't see MC as a building game, but more like a survival game. That means when I get super powerful items and all danger has disappeared, the game just stops being fun.
The suggestions in this thread don't seem like sinks of energy, more like investments. The idea I had for a fun game is adding lots of dangerous mods, like Hostile Worlds and Aliens vs Predator. I want to pair them with, say, RotaryCraft and have the arrow gun, force field, laser gun etc actually serve a purpose. (I should add, I've never got to late game RC so I don't know that they're completely useless, but to my knowledge RC doesn't add any dangerous mobs.) I want a good reason to make a well defended base!
I think it would be useful to consider survival mode MC as a survival game first, rather than a building game with restrictions, but it's important to remember MC is different things to different people. Some people will be happy using their energy to make building blocks for creative-style builds. There won't be a simple recipe to make the game fun for everyone once they're bored with vanilla.