1. Ainley Master vs. The Valeyard. One thing I wished there was more of in the Ultimate Foe was interaction between the Master and the Valeyard. So how about one where it's just them, no Doctor (save for the Valeyard.) The Master stumbles onto some plot by the Valeyard, whose plan threatens a plant (maybe Earth) and the Master resolves to ruin it, solely to prove that he's the Valeyard's better.

2. Return to Androzani: a sequel to the Caves of Androzani, with more corporate intrigues. My only real ideas for this is a) a group calling themselves the Sons of Sharaz Jak, wearing similar outfits to his, and believing he was fighting Morgus for environmental reasons. Their leader claims to be a still-living Sharaz Jek, but is eventually revealed to be Jek's Salvateen android; and b) due to the traumatic regeneration, the Doctor fears returning to Andorzani minor, and who wants to leave it as soon as possible,.

3. Dimensions in Time novelization: there are fan novelizations for Douglas Adams and Eric Saward's Daleks, but I'd like to see one of Dimensions in Time. This could go in several directions: a serious rewrite of Dimensions in Time, turning it into an epic; or a more comedic style that just transcribes the event in a Terrance Dicks style.