Multiple times I wished you could have 3 Society Traits, and I'm not the only one because there are mods for that.
But this is more than that, it's a great design space for the "create your own/storytelling" aspect of the game, that seems to be a core design choice of the devs.
An example of the top of my head would be - I know people wanted to have their faction be Undead from the get go (because there are mods of that as well) . So what If you could start w Undead, but without the added good stuff of life steal or others, and instead w more downsides, from the lines of Undeath as a curse.
#Negative Society Traits ( Allowing 3 Society Traits, but at a Cost )
static pendant
ancient reef
I wouldn't even mind the lifesteal bonus. Negatives are huge, especially when other players can build reliable counters from the time they meet the race,.
polar cairn
I mean, lifesteal is already a society trait you can start with. So I don't feel like that's gonna be much of a barrier to undead starting tranformation.
On the topic of 3 Society Traits, I'd like it. Not too worried about the extra Society Trait in itself, but the extra 1 affinity solves so many of my builds that can only work with a Dragon Lord ruler, which often doesn't work thematically.
timid charm
How about a 3 (maybe 4) point utility form trait named "cultured" or "gifted" that allow the race to pick a 3rd society trait.
opal pumice
Personally, I envision something similar to Stellaris, where you start with two Civics (a bit like traits), but as you progress in technology, there's a research option that unlocks a third Civic to choose
While Age of Wonder 4 has a very flexible and non-linear ''tech'' tree, the only approach I could find would be a bonus research (like the research for Magic Victory) unlocked in the first Tier 4 Tome, allowing you to choose a third Society Trait in the mid- to late-game.