Magnussen rapes and abuses in any way he finds interesting both Mycroft and Sherlock. Both are traumatised, but maybe Mycroft copes better, idk. Neither Holmes knows that the same has happened to the other or maybe Mycroft learns about Sherlock, but manages to hide his own terrible experience.

I'd love to see them coping each in his own way. Maybe at first John is oblivious, but then he somehow learns what Sherlock has been through and is supportive.

I don't mind John/Sherlock (preferably non-sexual or, better, unresolved. Pre-John/Sherlock is fine, too). I also don't mind some unresolved Mycroft/Greg, possibly ruined by Mycroft's ordeal. Those are not necessary though.