It can also be 2 revisited primals and 2 "new/meddler" boss like it did this tier ; so that we'd fight the last 2 primals in final tier.

Although we did fight Levia and Titan back to back so if we stick to the plan, we're supposed to do them all in line, it's easier story wise to have 2 bosses going according to plan and 2 "meddlers", twice, than the entire middle tier being the 4 remaining primals (going according to plan) and the entire last tier made of meddlers / out of the hat whatthef*ckery. If I had to write it, I would find it harder to give sense to 4 out of nowhere meddlers back to back.

Next tier is i480 (n) / i500 (s) since i500 will be the new cap.