We know from producer comments that the team members who came and went during the five year gap were: Tempest, Aquagirl, Robin (Jason Todd), Troia, Sergeant Marvel and Lieutenant Marvel (assuming those are the YJ versions of Mary Marvel and Captain Marvel Jr.)
Of those characters, Tula and Jason are known to be dead. I'm assuming the "one will die" is one of them.
I think it's been pretty clear from Tula's the one that will die since Tim is already Robin.
Yeah- knowing where things were at the start of season 2, I'd say:
Kaldur "falls" into the hands of the light
Nightwing rises to the mantle of leader.