A bit of a wild idea.
The whole child-soldier thing really bugs me about the ST. In the OT - I felt the battle between the empire and the rebels was fairly morally unambiguous. The empire was evil - the people fighting for it were likely culpable. But the element of recruiting child soldiers in the ST really makes me feel bad for the Stormtroopers and Officers who are killed.
Finn’s plotline as a defector - along with the other defectors that emerge to join them really made me think that perhaps the ending of Rise of the Skywalker could have been a mutiny? The cycle of violence between Republic loyalists and Imperial Supremacists might have been in mutual opposition to the Emperor and the Sith. It’d take some setting up - like some lie or truth unmasked that really sends the First Order troops over the edge to maybe turn on the Red Armoured Sith Troopers and older officers.
No idea how this would come across or work - or even the shape such a plotline might take.