^ Well, Carole Ann Ford has appeared as Susan in a few recent Big Finish audios, so presumably she could reprise the role herself if the need arose and she was willing.
I think that's a problem with these characters we haven't seen in awhile. The Doctor is changed. Why haven't they?
I would love to see Kate O'Mara return as the Rani. But the last time we saw her was with Seven. With the Time War, combined with her dangerous lifestyle, it's unrealistic (at best) to think she hadn't regenerated in all that time.
Susan, we haven't seen (ok, not counting Five Doctors) since One. Again, it's not realistic to think she had gone without a regeneration and she's still in her first body. She went through the Dalek Invasion, and who knows what else on Earth, while the Doctor has gone through ten regenerations.
As for Romana, there's zero chance Lalla Ward (or the superior Mary Tamm) would still be Romana on screen. She's in E-Space. She's back on Gallifrey as the President. There's that pesky war, and the return of Rassilon. Most likely, she's returned to E-Space, after having regenerated at least once, if not several times.
Unless we have a story that's early in the guest's timeline, the original actor that we know and love would be playing them. And without that original person, what's the point?
Other than nostalgia, there's no reason to bring back Susan. And if it's not Carole Ann playing her, why bother?