The Eighth Doctor, Liv, and Helen

With Dark Eyes, Big Finish was experimenting with new formats, and in the end established a precedent, at least for Eighth Doctor stories: An epic with a single title, composed of four boxed sets, each with four individual one-hour stories, released twice a year. It became clear pretty quickly that a sixteen-part epic was not a great plan, as you can’t maintain suspense for that long, so the boxes became a balancing act between the series arc and stand-alone stories. At the moment, this era consists of the Doom Coalition, Ravenous, and Stranded arcs, followed by a few boxes that aren't part of larger arcs. Stability is provided by having Liv present for this whole era, and Helen around for most of it. These were also released after Big Finish obtained the rights to use new-series characters and concepts, and you can see that creeping in slowly over time.