Well how about the Hellfire club take part somehow?
And a Deathstroke-type villain would act as the new enemy for Nero, if he murders Kyrie right in front of him. That guy can work for the Hellfire club.
They can work to serve another demon lord more powerful than Mundus or revive Mundus himself.
Give Mundus a power upgrade and only Urizen was stated to be stronger than Mundus but Mundus and the hellfire club with using Urizen's dna to make Mundus stronger.