I've got two suggestions for today:
Rubber Sheet:
Currently, the rubber sheet is very underused and not that useful.
My suggestion is to make it nullify ALL fall damage aswell bouncing the player upwards at a HIGHER velocity (about 50% more) than it has reached the sheet, allowing players to propel themselves upwards using the rubber sheets, like trampolines. Holding shift (crouching) would completely stop the player from bouncing.
RTG cart:
RC-IC2 integration item, that simply puts a RTG in a cart, allowing it to give off power to RailCraft electric locomotives.
Possibly a single core nuclear reactor cart aswell
Who doesn't want a nuclear-powered locomotive?