This was a pretty excellent listening experience overall. Yes, Once and Future is very much a fan service story.  It's all about the "X meets Y" trope so many critique Big Finish for.  Yet I must say I have largely enjoyed these audios so far.  Two's Company was a little rough but it wasn't even that bad, and then you have stuff like this wonderful little project that I had a lot of fun.

The First Doctor, or, maybe the Tenth Doctor, finds himself on a planet seemingly, but impossibly, recreating War of the Worlds.  It turns out it is all Missy's doing as she too, is experiencing some form of degeneration.  Also, the Paternoster Gang are here, and they are a weirdly welcome and delightful part of the audio.  These characters were always good in moderation up until Moffat got a little too self-indulgent with them.  It's really fun to have the Tenth Doctor interact with the gang and Strax has some very fun scenes here.  I especially enjoy him describing the Thirteenth Doctor as a "little boy with yellow hair."

The real showstopper is Missy though.  Her meeting the Tenth Doctor was something else.  The two have this amazing back and forth and you can tell they are ad-libbing like crazy.  It's so good hearing these two just fall into these old roles and play off each other perfectly.  It easily makes up for the more clunky aspects of the story that turn up here and there.  The doctor remarking how he "wanted to go" at the end feels like an example of said clunk.  It is somehow a cute moment for his character, weirdly satisfying, and took me right out of the story all at once.

I would love for Michelle Gomez and David Tennant to get a box set together because this was something else.

This one was always going to be my favourite Once And Future release, as it features both my favourite Doctor (David Tennant) and favourite Master (Michelle Gomez). It does not disappoint. The pair have so much chemistry that it makes you wish that this Doctor and this Master had met on TV. Whether it's the Doctor's bafflement that Missy would turn a deserted planet into War Of The Worlds because she was bored, or the pair having to deny that they are a couple, they're an absolute delight together. It's interesting that Missy also got hit by a degeneration weapon. They seem to hint that it was a past Master who degenerated into Missy, rather than Sacha Dhawan's Time Lord, so I wonder which Master it was? Derek Jacobi's, perhaps?

The Paternoster Gang also work surprisingly well with the Tenth Doctor, especially Strax. His wish for a pony felt like a fun reference to the horse riding Sontaran in War Of The Sontarans.

The whole story is as though the Tenth Doctor has been plonked in the middle of a Twelfth Doctor Series 8 episode, particularly as it plays with the idea of a story the Doctor knows to be fictional turning out to be real (sort of), like with Robot Of Sherwood and Last Christmas. It's an entertaining mash-up that works remarkably well.

This one surpassed many of my expectations.
First of all, after three stories we finally get more hints for the puzzle.
Second of all - yes yes, David Tennant is here and he is great, but that's not him I was so excited about. Not him, but Stephen Noonan as the First Doctor. I was worried that the original three might not get time to shine, yet I'm actually happy that the First Doctor (one of my favourites) was in the beginning of this story and leading the plot for some time. I was sceptical about Noonan's performance at first, but after this one I can safely say that he is amazing as the First Doctor.
Missy... she is wonderful. I didn't get to her in the series yet, but she is glorious and I can see why people love her so much. And the reveal about her part in this anthology was also great.
The Paternoster Gang was really funny here, all of them got time to shine. And the original character Jesamy Moore was interesting, although she kinda fell into the background near the end of the story.