Susan's departure from the Tardis at the end of The Dalek Invasion of Earth was the first departure of a comopanion, and in some ways the least satisfactorily resolved of all; what sort of life does she face, presumably one of the Doctor's own race, but living with humans for the rest of her life? (When she pops up again in The Five Doctors we are told nothing of what she has been up to in the meantime.) The 1994 radio play Whatever Happened to Susan Foreman? had her wandering back to the twentieth century and becoming European Commissioner for Education, but it is not a serious attempt to contribute to canon. Big Finish tried a bit harder with Marc Platt's An Earthly Child at the end of last year, which brought Paul McGann's Eighth Doctor back to Earth decades after Susan's departure, and guest-starred McGann's son Jake playing Susan's son Alex, but I
wasn't completely convinced.
By contrast, I loved John Peel's Legacy of the Daleks. Peel is a bit of a guilty pleasure for me - I rate his novelisations of the black and white era Dalek stories very highly, and appreciate his attempts to wrest continuity and character from material which is not always promising. Here, he has Susan trying to manage her relationship with the aging David, putting on make-up to appear nearer his age when they are together in public, in a post-Dalek England which has become a patchwork of feudal fiefdoms. Throw into the mix not only the visiting Eighth Doctor, but also the Delgado!Master attempting to Take Over The Universe by reviving the Daleks and stealing their tech, and the book ends up pushing many of my fanboy buttons, ending with hope for Susan and a prologue to one of my favourite TV stories. Best Eighth Doctor Adventure I've read for a while.