So, after looking at some of the posts from other users, I have to acknowledge, I agree with a lot of the suggestions that have been made since my post (or even some of the ones prior that I just didn't look at until now).
Metallurgy looks pretty beautiful, so that's definitely a really good idea. And it's already supported in 1.4.5 through Forge, so all you'd need is just the author's permission.
Thermal Expansion definitely adds a lot more fun technology to the game. It's different enough from IC2 and even GregTech's machines that it wouldn't really cause too much redundancy, but adds even more possibilities to the pack -- which I'll elaborate on at the end of my post.
Logistic Pipes adds a lot more potential to Buildcraft just like the Additional Pipes extension, so both are worth adding imho. In my eyes, any major extension to an existing mod (i.e. Buildcraft) should be included with it. After all, GregTech was included and just adds on to IC2, and Forestry essentially just extends a bridge between IC2 and Buildcraft. Both are worthwhile.
Finally, yeah, weapons are definitely lacking. There's still some toys to play with like dynamite, the weapons from Twilight Forest, etc., but I'd definitely say that Balkon's Weapon Mod and something like Altriacraft or Swords++ are worth adding. At least Balkon's, as it gives a lot of other possibilities.
I'd say now though, outside of the mods that are already going to be added (RP2, EE3, Xycraft), that Metallurgy would be my top suggestion. I really like the vast amount of ores that adds. I know it just means you'll be mining forever, but that's one of the major appeals to Minecraft to me.