Watched the special - the "alien of the week" plotline was pretty predictable from the get go and it was nice seeing the DoctorDonna again. Donna was always an underrated companion. The "message" they worked into the special was pretty heavy handed, though. Like they could have made it work with better writing, but it was Supergirl on CW levels of subtlety so it felt really forced.
I'm assuming that cryptic line about "2 hearted creatures are rare, I'm going to have to tell the boss" is going to be what the 2nd special revolves around. The Master maybe? Although they've used him a lot lately, so not sure it would fit to bring him back so soon unless they plan to recast him and use him in the next series.