A pretty delightful experience overall.  I really enjoyed all the major elements of this audio, from the exciting start showing off the degeneration to the ultimate focus on the schemes of the Monk going up against the Fourth Doctor, Sarah Jane, and UNIT.  I get how some people might object to the "fan service" nature of this audio, featuring a number of cool cameos and performances throughout, but I, as a fan enjoying something built mostly just for fans anyways, enjoyed this story nonetheless.  The Black Archive and its history with the curator was a neat little background detail.  Where the story is weakest is actually around the villains, who are kind of putting the Monk up to his scheme.  These aliens are pretty boring and have a pretty generic voice.  I don't really love them or their angle of being monsters from an until-now unmentioned story.  It just didn't really work for me.

Everything else did though.  This version of the Doctor meeting Kate, Oswald meeting Sarah Jane, yeah, sure, why not?  I had a good time, I was entertained, I was interested, and that's all that really matters.

An exciting action-packed opener to Once And Future. This one involves the degenerated Fourth Doctor chasing the Monk through time and becoming embroiled in an alien invasion by creatures he defeated with Sarah Jane back in the 70s.

The Hyrinth are decent enough monsters, although I'm not sure they'll catch on in the same way as the Sontarans or the Ice Warriors.

The highlight is hearing the Tom Baker era and the new series collide. Hearing the Fourth Doctor meet Kate Stewart and Osgood offers something fresh and new.

We also get some lovely character moments between the Fourth Doctor and Sarah Jane. I do feel for Sarah with the Doctor refusing to take her back home. As the Doctor's best friend, that must have hurt.

Oh boy, what a start to the anthology!
I can't even think of anything to change in this story, it is a perfect start for such an anthology.
The Doctor is staggering between incarnations and neither he, nor anyone else have any idea what is going on, and it is perfect.
Everyone in this story is acting greatly, making the characters alive. Tom Baker is still blasting as the Doctor. Rufus Hound as the Monk - good grief, whenever he is present, I'm smilng from ear to ear. Sadie Miller is virtually indistinguishable from late great Elisabeth Sladen - her voice is perfect for Sarah Jane Smith, and her storyline here is interesting. Jemma Redgrave and Ingrid Oliver are also amazing as their respective characters. I'm beginning to like Osgood.
The only things I didn't like is how does the Monk know about the Doctor's daughter? Also, if this is an anniversary, I think the Hyreth should've been swapped to someone familiar to the audience, although the Hyreth storyline was kinda tragic, not gonna lie.