Rubinon said:

I'm pretty sure that the R'hllor command crest was added so that Baratheon could be split in three just like the other houses. The rules state that any commander can be used with those crest. That being said, the ally expansions have been made so that they can be used with any house ( may not fit with the story). I imagine what ever ally expansion is next will include a new command crest.

 I think it is the strongest (only?) evidence of what might be coming next, but that hardly means it is definitive. I'm still reluctant to place Tyrell into the allied forces category. They have their own agenda and it only aligned with Baratheon's for a short while. Maybe we could get something of the Pirates and Mercenaries flavor in the same way The Brotherhood was released. That would also be a workable allied force for Baratheon, similar to Tribes of the Vale. Still, if I were betting I'd say FF tipped their hand for the next one. It would just be weird for that to be there otherwise. Baratheon just doesn't have as many good options for allied forces that supported them in multiple battles over the corse of the books, with the possible exception of Salador Sann.

The real question is who would even work as a Greyjoy allied force. I mean, does anyone else even like them? :P