[I looked ahead and the next Target fourth Doctor release is Androids of Tara in July and they haven't announced who'll be doing that one (hopefully Mary Tamm since it was very much her story).
The reader has been announced, it's John Leeson.
Androids Of Tara is a new adaption like Stones Of Blood. I suppose they might try and do Pirate Planet as there is a rough Key To Time theme going, in the near randomness of it.
Armageddon Factor was a thin book though. Be cool if Bob Baker could do a new adaption.
The producer of the Shada book revealed he is very keen to work with Lalla again.
It's not to do with rights, its just what ever order they feel like doing them in. Just counting the on my CD cabinet there's already passed 40 of them, so it could hit the 1/3 point next year.