You'd probably have to bump the version back to 1.6.4, seeing as there are no stable releases of any mods since then.
That said, I like Railcraft and Computercraft.
For those who don't know them, Railcraft adds to the transportation side of things. You get track junctions and switches, some fantastic control tracks, the ability to link up minecarts turning them into actual trains and put a locomotive in front, or signalling for a greater order in a rail network. Computercraft adds programmable computers that can interact with redstone (Project Red might be a good addition to it with its bundled cables, replacing RedPower) and Turtles, a sort of computer that can also move around by itself, break or place blocks, etc. There are also printers and networks and stuff.
Or if you'd rather go for something exotic but not too outlandish, Dimensional Doors. It adds some mysterious-looking doors to world generation that take you to pocket dimensions full of unknown. You can also make them yourself and they take you to an empty personal dimension full of void. It's supposedly at beta stage but I think it's pretty stable.
Or maybe Galacticraft, if you want to have a TWHL space program. You can go to the moon or mars or set up a space station.
I was going to suggest Modular Powersuits too but it hasn't been updated since 1.5.2.
I'm not suggesting to have ALL of them, these are just the mods I've played or read about lately that look really interesting but not too over the top like some I've seen (the FTB pack comes to mind). This should give people some ideas
