I would happily see the Black Guardian and White Guardian return, as what type of story or how to integrate them. Well as an earlier post has already pointed out, a series arc would be ideal for a return for these two characters.
A modern reworking of appearance minus the bird probably , would be easy enough.
As for the Black Guardian Trilogy back in the Peter Davison era, I always felt it was a little underdeveloped and could have played more of an integral part of the stories. There certainly was plenty of scope in a story like Terminus, where little seemed to happen in a couple of the episodes. It would have improved that story no end if done sympathetically as it were.