If we could merge Stars, Maelstrom and Distant Stars and some of Zombie's Fallen into a single ubermod...
You would hit the 2gb and be adding way too many different things. I still think Maelstrom, Zombie's Fallen, Babylon 5, etc that add new races should stand on their own.
However, what I would like to see are all the changes to the original SoaSE races compiled into one SoaSE ubermod. Here would be my ideal version:
Use the Stars mod as a base with all the extra research from Stars (especially the engine and phase travel speed upgrades, and doctrines) and the Stars titans (until rebellion), and then add the following on top of that:
Everything from the 4x mod: the new diplomacy system, the Rebalanced Trade/Refinery/Resource Focus mechanic, the balance fixes to Advent, the racial abilities, the faster culture system that also buffs allegiance, and the 4x version of the random encounters. Also, keep resource asteroids at all the planets and make them look like moons (Stargate Invasion mod). What annoyed me about Stars was that you had to rely way too much on asteroid belts. This isn't a problem for Stars maps, but if you want to play original Sins maps with the Stars mod you were pretty much hosed.
Lots of things from Distant Stars: The extra strike craft, planetary abilities, pirate stuff, and especially the planetary defense system.
A compilation of all the various planet mods added in (including all the Stars planets but with more resource asteroids, Maelstrom's planets without insane credit and resource rates, Celestial Bodies, what Axeldude is currently working on, etc combined with the resource asteroids turned into moons and the unexplored gravity well graphic from the Stargate Invasion mod).
All the various graphics, sounds, and explosion enhancement mods.