Other than for fancy affinity-related leader animations, modding and adding factions and leaders with varied personalities & preferences should be pretty routine for our fine community of Civ modders? It was in Civ V and I assume it will be for BE too.
Am I right or over-simplifying what it would entail?
Having done a couple mods for Civ V, the xml coding for civs is straightforward and relatively easy to put together (although it's restrictive in what you can do since it depends upon what Firaxis has allowed in the xml code).
It's the art, voice over, animation stuff that's more time consuming.
For quicker mods, you simply use pictures instead of animations for leader portraits. Forgo voice overs by either not having any or by "borrowing" the VO of another faction.
On the bright side, there aren't any unique buildings/units for the Sponsors so you don't have make art resources for those in BE.
The downside is there are more animations for each Sponsor due to affinity-based modifications.
If I do a Civ:BE mod, I'd probably just go the fast route and ignore animations in favor of a picture for leaders and just borrow the VO from another Sponsor (or perhaps Civ depending on how much overlap in VO there is between the two for diplomacy purposes).