What i'd allways dreamt of makeing was a space mod, a mod that combines all other mods with a end game goal: building a spaceship. Have it bee super expensive and use items from pretty much every mod.
Step 1: build a hull
Step 2: select a propulsion system (ion thrusters, fusion powerd etc)
Step 3: supply it with insane amounts of power (like 1billion eu and 400 million mj)
Step 4: launch
Step 5: land, this is another place you could combine alot of mods. Say have a forestry farm producing oxygen have a air tank MPS modual. Have a buildcraft moon quarry. A Turtle powered asteroid minning ship
the possibillitys are endless.
I also think there should be new ores and dusts there, like helium-3
I think this would be especially fun on a server having a space race of sorts
Sorry for any spelling error it's hard typing on a phone