This is an... awkward and specific but curious question.
What faction would you like to see expanded on, either in endless legend or beyond?
Perhaps (if there ever is an Endless space 3 or something) in a future game? Or with more lore / impact?

Honestly, on my own part would I ADORE to see more lore / info regarding Cultists.
Perhaps it is just me and my weird tastes, but their design, their core function and the lore we see REALLY intreguies me.
The whole "manequin" thing and conscripting people to their cult makes me really intrugied.

I doubt we will see them again, due to how nieche they were, but hey... a man can dream, right? :D

I always thought the Nakalim were the Cultists (religious, made up of other converted minor factions, female leader has an extra set of mechanical arms and some resemblance) in a similar way that the Cravers are like the Necrophages. But I don't know if they are literally the same faction just in the future, they probably aren't officially. Maybe some of the Cultist lore will be expanded on further in the upcoming DLCs, I guess that remains to be seen. 

I would like to know a bit more about the Morgawr personally, I think they are really cool and not too much is known about them although the fan theories I have read are interesting.

I just want Endless Legends 2 to be made with all the best new features from both Endless Space 2 and Humankind mixed in. "Good" fantasy 4x are rarely made for a long time now which is a shame cause I feel there is a real potential for someone to make a fantasy game and go big with it. Like if you look at the market the only ones that are really good is Endless Legends and Fall from Heaven 2 (which is a mod that is better than most games). So the more the better!