There won't be more AR or BfA, let's start with that.
All other AR had asset's in game patches before being implemented, none of the options here do. And since it's pretty pretty likely most of the development is now on SL, I really doubt they are gonna take the time to introduce 2 more AR when there isn't even going to be a 8.3.5
As for the future, I do believe that LF Undead could be a possibility with all the lore introduced, and they could be pivotal on a light based expansion, but the lack of development as a group is worrying.
Interestingly, the introduction of the Rajani really opens the opportunity for them as they are mostly concept so far, most of their assets are reused, but conceptually they bring something entirely new, and after they fulfil their purpose, with much potential.
Saberon have a complete lack of set up. Sethrak will always be a good choice, it's a shame we didn't see them being introduced in Uldum to further flesh them out.