I'd LOVE for Nethercraft to be added. But sadly the mod was never updated since 1.2.5 .. It was one of my favorite mod back then.
I have to agree with this. I'd love to see Nethercraft, but I haven't seen an update in a long time. I haven't even touched it since beta 1.7.3, for that matter, so I doubt that'll ever happen.
On the other hand, Aether is being rebuilt from the ground up to support 1.4.5, and I
believe it's Forge compatible. If this could be added in, that'd be pretty awesome as an alternative.
Balkon's weapons mod, Mo'creatures...
Nether Ores would be cool. Gives more of a reason to set up shop down there.
I don't really like Mo' Creatures all that much personally. It's great for SSP, but I've never much liked it in SMP. But it wouldn't be too bad, I suppose. Definitely agree with Balkon's Weapons Mod and Nether Ores (since Nethercraft can't be added).
Personally I'd like to see additional Buildcraft Items/pipes, it adds so many useful pipes. The Valve mod might also be interesting, but I think Liquiducts from the Thermal expansion will do most of that as well (especially combined with additional Buildcraft items it really won't add much, I think).
The Additional Pipes extension for BC2, in my eyes, is a must. I was actually very disappointed when I scrolled through the items in NEI and couldn't find the teleport pipes anywhere. I used to use the conductive teleport pipes constantly for powering engines safely without having wires everywhere.