If there's a new primal in 3.5 part 2, and it isn't Rhalgr then I will be very surprised.
The whole first part of the story included him being invoked by name by the Ala Mhigan forces as they died during the (unknown to them) process of summoning, and then it being imprisoned. The final cutscene was even the Scions all discussing what they would work on in order to find a way to defeat it more permanently in order to not take the sacrifice made in vain.