Well ... I kinda got this headcanon over Halloween surfing on tumblr:
Like, if Mycroft and Greg would be living in the 18th century,
in the victorian era, they might be a vampire hunter team
And Greg is an ex-inspector, because a vampire killed his wife and now
he is on a vampire-killing spree and then he meets Mycroft
And Mycroft is of course a noble man with a terrible past, also involving vampires
So now they are a team and slaughtering vampires together and eventually start to love each other
Hah, well ... I don't know