Ed Beach was the one who talked about this very subject and who answered this in an October 2017 interview with Richard Scott-Jones from PCGamesN (
https://www.pcgamesn.com/sid-meiers...ongress-diplomacy-alternate-leaders-expansion):
"...Do Firaxis have plans to embrace it and add more alternate leaders?
'We definitely will lean into it more,' Beach tells me. He points out that this is a great opportunity for the game's vibrant mod community to make their own contributions, so it's not necessarily something that has to be done by Firaxis, 'but we have been doing some thinking about which would be interesting civilisations [for us] to add a second leader to, so I think you’ll probably hear more about that.' ”
So, to sum it up, Firaxis will do a few, but it is something mostly for the modding community's to do. This, of course, is now expressly obvious in the expansion because out of 9 leaders, only one is a second. According to this, and Ed is pretty much in charge of all Civ VI, I don't think it is in their plans to create a DLC pack of alternate leaders, however they will sprinkle one from time to time.