Okay, take this dynamic and then apply it to Eurus, Sherlock and Mycroft. Naturally, Sherlock would be the “superhero”, Eurus — the “supervillain”, and Mycroft the “mayor”. Eurus is a teensy bit more stable in this AU (but not any less smart) so she manages to break out of Sherringford’s much earlier on and walk free without Mycroft noticing. We know all Holmes siblings are masters of disguise, and she’s always back in her cell whenever Mycroft decides to visit, so it’s really not too much of an issue for her.
Of course, that doesn’t mean she’s forgiven Mycroft for what he did — but she has another brother, to whom she decides to reach out… in her usual fashion. Something akin to series 4 happens much earlier on, but at the end of it Eurus isn’t back to being locked up, instead choosing to team up with Sherlock and enact the dynamic shown above. Well, I say “team up”, when in fact they’re actively working against each other, of course; but they do conform to a set of agreements which will keep their little game going (really, Moriarty should have thought of that if he wished to not be bored.), such as:
No killing each other
No targeting Mummy or Dad
No killing of any close people to each opponent (though targeting them and hurting them non-lethally is, somehow, allowed)
Mycroft still hasn’t caught on.
