I think you’re outrageous putting Ace in your worst five, gridman. I’m less surprised that Mel’s there, but I have to admit (I’ll whisper this) I’ve never seen a Mel story. I have, however, heard her in several Big Finish plays, including The Fires of Vulcan in which she was excellent, so I rate her above many of the other companions. Similarly Nyssa, who never stood out on for me on TV, is far better when she and the Doctor travel alone in the audios.
Actually, while I’m off television for a moment, Frobisher‘s a classy companion.
Your suggestion of aging the Doctor reminds me of audio only companion Evelyn Smythe, with the dynamic of the relationship there dramatically changed (and successfully, I believe) with an older companion. I’m not sure I want to see an older Doctor again (unless it’s Bill Nighy, of course) and I think Martha could have been (and could still be) a really great companion. She was the one good thing in LoTTL.