The two obvious stories for me are Meglos and Nightmare of Eden - the former would've been a lot more fun in season 17 where it would've benefited enormously from an injection of Douglas Adams humour and the latter had serious intentions and some potentially gritty material that was utterly ruined by sloppy production and too much larking around. A more serious Nightmare and a less pompous Meglos would both stand in much higher regard.

What else would benefit either from a move into season 18 with the gloss and the attentions of Bidmead or by a move out of season 18 to a time more suited to its strengths?