I'm trying to remember the last time I was this bummed out after watching an episode of TV. I mean, I watch bad TV sometimes, but this wasn't just a case of "that was not a good episode," this was... watching this just made me so, so sad. Let's... dive in.
Cons:
I'm going to start small and work my way up, here. The Avengers Assemble of it all was fun, but it really sidelined Belinda and it even sidelined the Doctor in a weird way, where I wish there had been a bit less spaghetti thrown at the wall here? I remember feeling this way about last season's finale too, that there was just too much going on, and too much of it was connected to old lore that I don't understand or care about. I stand by that the bi-generation thing with the two Ranis was pointless and it should have just been a straightforward "Mrs. Flood is a Time Lady" twist without that added element tossed in.
The thing about Poppy is that I wanted to like this concept, I honestly don't mind the idea of the Time Lords being an infertile species and the Doctor "wishing" himself this miracle chance to be a parent, but it was too confusing and the stakes were therefore difficult to grasp. I think the plot cul de sac of "yay, Poppy was okay inside the box!" and then she vanishes from the TARDIS and now the Doctor has to do a big sacrifice, all should have been squished and curtailed. Like, what if it was just shortened so that the box is opened, Belinda is in there, but Poppy isn't, and everyone else has forgotten Poppy, but Belinda remembers, and then because of weird shenanigans, so does Ruby, and then it's Ruby and Belinda together making a case for this world missing one person and how that's wrong? That would have given Belinda more to do (more on that later), and cut through some of the weird pacing in the end of this episode.