I apologize for not having specific mods in mind; rather, these are more general suggestions for particular types of mods that I think would greatly benefit FTB. Hopefully more knowledgeable members of the community might have some suggestions, or (pipe-dreaming, here!) perhaps these ideas might even inspire a new mod:
- Mods that emphasize the need to travel, by encouraging exploration and---especially---geographical expansion. After mining that first handful of diamonds, too many mods aspire to give us everything we need without ever having to leave our house again. ExtraBiomes provides such gorgeous landscapes... but there aren't enough reasons to go exploring them. RailCraft provides absolutely brilliant transportation technology... but other mods remove the need to go anywhere, much less set up a railway network that allows back-and-forth travel over long distances (not that Vanilla Minecraft bursts with reasons to explore, to be fair). Examples: Mods with biome-dependent features (like Forestry's bees... before those advanced apiaries give you everything you need without leaving your house); mods that add/alter generated structures so you want to go back to them -- perhaps offering immobile artifacts/machines with unique functions, to justify rail/shipping lines to them; mods that emphasize villagers and village mechanics, increasing the need/desire to build a railway to the nearest village, etc.
- Mods that provide "sinks" for the massive quantities of resources generated by automation (esp. left overnight, w/chunk loaders...), to close the supply loop. Automation so often seems like overkill to me: A single Sugar Cane farm can generate an absurd quantity of Sugar Cane in just a few hours, to use one example. Some of the go-to examples of current almost-sinks include IC2's Recycler and Forestry's handling of organic materials, but neither of these are true item sinks---they compress/condense/convert items into smaller forms, but they don't use the items up. Examples: Mods that use up massive quantities of renewable items to do something, perhaps with an efficiency that isn't based on quantity but on rate of input (e.g. instead of "64x Sugar Cane = 1x Ubermabob", it would be "64x Sugar Cane per Second = 'Ultra Speed'"); Mods that can take a massive influx of resources and create a state of equilibrium, without constantly outputting something to storage; Mods with interesting and attractive end-game content requiring huge quantities of renewables to fuel and/or create.
- Balanced End-game Goals based on imaginative development, rather than mindless grinding. Perhaps the most important, but I left it till last because this one's a little obvious. Best counter-example I can think of is the Nether Star, which in my opinion was a terribly ill-thought end-game "goal". Grinding for hours in search of Wither Skeleton Skulls belongs in an 80's era JRPG (I still love you, Final Fantasy I), not the world of infinite possibilities that is Minecraft. As for "balanced", I point to ThaumCraft (2 and 3)---where everything has a cost and/or a risk associated with it (so I really hope EE3's scary-looking Redwater mechanic has genuine teeth). End-game content shouldn't be about turning Survival into Creative Mode---we don't need more flying rings... unless there's a good chance you'll fall---but about taking bigger, more expensive and more elaborate risks for proportional rewards. Examples: A mod that provides a more fun (but still difficult) means of acquiring Nether Stars (because beacons are great); Mods that provide more of those "functional structures" (the official term escapes me), where multiple costly blocks are assembled in a certain pattern to create a larger machine (e.g. beacons, Coke Ovens)---such structures strike me as especially good design space for balanced end-game content (I've long dreamed of a MystCraft-style tech mod that used rocket ships instead of books to take you to other planets, whose surface conditions have to be studied from high-altitude observatories so you know what equipment you need to bring with you to breathe...)
I suppose all of these suggestions carry a common theme: improving the end game. After playing a lot of Tekkit and FTB, I hate that "running out of things to do" feeling that so often comes after spending so long in a world. It always comes after a steadily-mounting stream of "I could do {insert project}... but why? I have nothing to gain from the fruits of my labor: no reason to travel, nothing fun to build, no need for more power." Sure, "because I
want to, just for fun!" is
an answer, but it's not the best one: "Because I
need to, so I can get {goal}..." is
far better. Moreover, the end-game is
especially important in a modpack like FTB, I think, because so much more effort goes into laying the foundations for a world: sorting systems, power grids, etc. Spending days and days setting all that up only to run out of things to do is a letdown, something that a collection of fun end-game goals based on building, exploration, and exploitation of the world would do wonders to improve.