3. The Zygon Subplot Wasn't Necessary
The return of classic monsters to modern Doctor Who can be a real treat. The returns of the Daleks and the Cybermen, in Series 1 and 2 respectively, managed to reinvigorate old villains as well as give them some new modern context. We had new reasons to fear these monsters, because they had modern reasons to come back. However, this sort of thing works best when you do it gradually and not too often without real purpose. Even if there were a perfect reason to justify their return, we didn't need the Zygons to return, because we had a perfect story to tell with The Last Day of The Time War. The Moment/Rose still would open the portals across time, luring 10 and 11 into helping The War Doctor not to end The Time War, and the adventure could have eventually become The Doctors trying to fight The Time War to a Stalemate. Realizing a Stalemate would be impossible, The War Doctor would still try to use The Moment, and the three Doctors could still very easily work their way to not using the device. But instead of the battle filled fulfillment of a promise made back in the Davies days, we were treated to reddish creatures covered in suckers with the ability to pull a Loki on command. Creatures that can copy anyone's likeness aren't new, but seeing a Time Lord breaking down to the point of moral compromise is something that is much more exciting.