Assuming they will release new apex bosses, I guess the best way to introduce them is to fit them on weekends? That’s 3 more slots, at least we don’t have to get rid of mortem for now. Or have more than 4 bosses in a day making it even harder to have the boss you need in range.
Another suggestion would be replacing the epic (or even legendary) bosses with apex bosses, since the epic dnas are the easiest to get among the 4 rarities.
Or we can vote a creature out to be replaced each time before they add a new raid boss