I think my favourite thing about season 10 is that it’s like the Doctor and the Master have come full circle back to the Time Academy. So much has happened between them. They’ve changed a lot. So much has gone on in their lives. But when it comes down to it it’s just Theta Sigma and Koschei back together at an educational institution.
But it’s a little different now. They’ve grown a lot. The Doctor’s a professor now (and you can bet Missy gets a kick out of that. “You? You, Thete, a professor? You barely passed back at the Academy and they’re actually letting you teach? HAHAHAHAHAHA!”) and Missy has to be kept locked up, but it’s not so different between them. It’s easy to slip back into old habits (nicknames, attitudes, friendship). The Doctor itches to break her out and run off and get into trouble with her - he would if it wasn’t so important, for her own safety, to keep her locked up.
Missy said she wanted her friend back. Well, she’s got her friend and a Time Academy AU. I’ve had complaints about Moffat before, but I am honestly so grateful that he’s given us this before he leaves. No matter what happens the rest of this season and in the future, the Best Enemies fandom is always going to have this part of their timelines to fill with fics, fanart, headcanons, etc. We’ll always be able to come back to this part of their lives and have the Doctor/Master ship. Whatever happens - I’m not naive enough to believe there will be a happy ending - let’s not let it ruin this time. Let’s always enjoy the Vault Era.