The Short Trips Rarities range is producing a unique new format to tackle for all of Big FInish's stories. Though there's so many in both print and audio to be a hassle to try and go through entirely, it'd be something to experiment with in some of the more imaginative stories.
For me, I've just learned about "The Deep" from Volume 1 of the audio Short trip collections, and my god does the idea of the TARDIS turned into a whale sound bonkers.
Simon, I know you don't do requests, but may I make a suggestion? Personally I would really like a Big Finish-style cover for the "Pickwick Talking Books" audiobook of State of Decay read by Tom Baker. It was a unique "novelisation" by Terrance Dicks and was very different to his own "true novelisation" (recently released by BBC Audio).
I have some very tentative ideas floating around. The problem being I'm dedicated to a commission project for a couple more weeks and was banking on slotting the Last Adventure (and the forthcoming Third Doctor stories) in around the original September release date...