I don't think devs are planning to add any multiple leaders per civ. I recall interview for civ5 in which they described painful process of adding new leader (it takes weeks to do one leader's diplo screen) and while civ6 at least doesn't have 3d backgrounds, it has even higher quality of leader models so that compensates simpler background. On top of that each leader needs one more unique bonus and one more unique agenda (both of them bug free, balanced, researched etc). Don't forget usually unique theme per leader and of course an entire dialogue for him (in native language).
In civ4 multiple leaders were possible because their diplo screens were very poor when compared with civ5, and they had no unique agendas or abilities (each leader was just a mix of preference wighs and two general traits taken from universal list).
Now it's simply not economic to waste time on all this new high quality stuff for a leader (fun fact, devs said
75% of costs and effort of civ5 civilisations were spent on leaderscreen) while you could make entirely new civilisation instead.
Thus, I am almost entirely sure there won't be any Firaxis-made multiple leaders per civ in civ6, and in fact I prefer it this way - I wanna see more exotic civs, not multiple leaders for old ones.
The toned-down backgrounds of Civ6 leaderscreens and the recent confirmation of leader-related uniques just reinforce the probability of having multiple leaders.
Toned down backgrounds save minor amounts of effort compared to the fully voiced over, animated leader model (with its quality much higher in civ6) and now each leader also needs unique ability separate from civ + separate AI agenda behaviour, so actually I think making a leader for civ6 requires slightly more effort than for civ5.