After some heavy hinting from Nick Briggs on Twitter, Big Finish has announced The Further Adventures of Lucie Miller. Details are in the news item Lucie Miller returns!
Quick summary: four stories in a July 2019 boxset, set in the gap between the first and second series of the original Lucie Miller Eighth Doctor adventures.

The Further Adventures of Lucie Miller details

What we know so far:

Four new adventures featuring the Eighth Doctor and Lucie Miller

1. The Dalek Trap by Nicholas Briggs

2. The Revolution Game by Alice Cavender

3. The House on the Edge of Chaos by Eddie Robson

4. Island of the Fendahl by Alan Barnes

More story details to follow.

Note: These adventures happen after Doctor Who: Human Resources

Written By: Nicholas Briggs, Alice Cavender, Eddie Robson and Alan Barnes
Directed By: Nicholas Briggs

Also Scott Handcock is producer and it will include the usual disc of extras.