I had thought of these things that you guys mentioned...I don't know whether we're going totally overboard or not.
I was wondering about the one of them being trans and being identical twins. I wondered for a while whether that could happen (Teagan and Sarah are twins, and both or gay, so wouldn't both have to be trans?), but then I remembered that Laverne Cox (a trans woman) has a twin brother (cis male) who played her character on Orange is the New Black pre-transition....so it can happen for one twin to be trans and the other not
If they wanted to make them identical twins, they could have Martin play her as a joke, or have it implied that one of them is trans. As someone who reads Translock fanfiction (with either Sherlock, John, both, or other characters as trans), it could be cool to see that become canon...though it would probably have to be Harry out of the two siblings if anybody, or else, with medical advancements as they are from what I understand, John couldn't father a child. At least not in a "traditional" way. But trans or not, Martin could still play both if they so wanted to go that way.
But it's still an interesting idea.
I remember also seeing someone's theory on Tumblr a long time ago that Sophie Hunter might play Harry if the character were to appear (if for whatever plot reason we had that were to be possible). That was a Johnlock becoming canon theory more than anything else, though, so I don't know how well it hold up otherwise.
It has already been mentioned that we might not see Harry at all because the equivalent character never appeared in the original stories. It would depend on how much the writers want to stay with canon in that respect. They have deviated or changed things in some ways, and have introduced other new characters, like Molly, Anderson, Sally...so maybe they could have Harry appear, too. They've mentioned her more than once at this point, at least once per series, so that could be leading somewhere.
