this post is for modders too get ideas from, since i know NOTHING about modding. i couldnt even mod OBLIVION correctly. if youve modded oblivion, then you now know how terrible at modding i am.
now in sins, there are only 3 factions.
tec--good economy
advent--powerful culture
vassari--uber combat
i played galciv, also made by stardock, and loved how there were like, 10 factions. i think sins needs more.
for a few examples:
the vassari's so-called "unseen foe"
a race that has the ultimate tech tree
a race that has more but weaker ships
a race with fewer but more powerful ships
some organic type race (similar to the SW yuzhaan vong, but more fleshy and less rocky)
one who has this god-like character that imbues his servants with all sorts of really weird powers that would be great to micromanage (similar to advent, but not like telepathy)
one that focuses on a complete SPEED----> overhaul
one that specializes in cloaking
i hope these idea's get someone going with a big faction add on
good luck