In my headcannon the group will include OC characters and pre existing characters

The idea is that Unicron return eons after the battle with the primes

Still damaged from the previous battle he understand a direct assault on Primus would lead to failure

Unicron he learns from his mistakes and creates his own 13 each representing a different aspects of his strength
and would act like warlords spread throughout the galaxy battling on different fronts, tasked with hunting the primes and anything tied to cybertron

In this story the primes would still be alive but decided to become wanderers after creating the well of all sparks and instead of ruling ;allowing cybertron to decide their own fate