I've been playing with IC2 and Buildcraft 3.x.x, and they're good mods. However, I would like to take them a step further. These modifications mostly apply to IC2, but may be useful for Buildcraft as well. Here they are:
1) Geysers. Personally, I find the idea of feeding a Geothermal Generator 
lava odd. A real Geothermal Generator runs off of water boiled with the Earth's heat. In this case, I would suggest having a water pump-gen combo. After mining for a magma deposit or a certain depth underground, you could have the generator take water from the pump, run it down a pipe (similar to the miner pipe or pump from BC) and back up to generator to produce EU. The generator GUI would have both a cold water gauge and a hot water gauge. Cold water would be the water from the pump it has stored. Hot water, in sufficient quantities, would be taken from storage to produce EU at about 10-20 EU/t, depending on the quantity of water in the machine. In addition, if separated lava/magma and water are within 4 metres of each other, the water would boil. These would spawn in geysers to the surface or could be created artificially, as stated above. The boiling water could then be pumped into the generator and used as-is.
2) Carbon Fibre Blocks. Pretty straight forward. Just take 9 carbon fibre meshes and arrange them in the crafting table. You would get a block with a similar texture to black wool, but with a strength just over obsidian, say, 72.
3) E85. Again, very simple. I don't know if this has been implemented already, but it would go like this: Take an empty fuel can. Fill it with 4 Biofuel cells and 2 Petroleum cells. This would give you one can of E85, with an energy density between that of regular petroleum fuel and ethanol.
4) More power sources. This one is inspired by Sid Meier's Alpha Centauri. You would have a Fusion Torus (20 EU/t), which would run off of hydrogen (water cells put in an Isotope Manipulator, which would be used to make Hydrogen Cells and U-238 Cells, the safer version of the re-enriched Uranium (U-235) Cell with 3/4 normal output). There would also be a Quantum Filter System, which would run off of EU with a 1/10 chance of QUADRUPLING the power put into it. it would accept up to 512 EU. Then we get to the holy grail of generators. Meet: THE SINGULARITY HARNESS! Running off of artificial black hole cores made of uranium ore and UU-matter, it would produce a whopping 2048 EU/t!, which could be fed into 2 cables. It would be very expensive to make and run but would probably be able to power an entire city! Most cities I make require some 10 nuclear reactors to run.
5) Lithium. This would be mined as an ore, extracted, powdered, hydrated and used in the production of batteries instead of Redstone (which I view as ferrous silicon)
Recipes:
Carbon Fibre Block:
( =Carbon Fibre Mesh)
Fusion Torus:
( =Lithium)
QFS:
Proton Shielding (Harness component)
Artificial Singularity:
Singularity Harness:
:Solar Panel:
:Energy Crystal: :Empty Cell: :Energy Crystal:
:Solar Panel: :Iridium: :Solar Panel: : ( Solar Panel: =Proton Shielding)
Thank you for reading this entire post. This is my first time here, so feel free to comment with constructive criticism. Again, Thank You. 
