I know that it wouldn't run in the time line but could we ever see a previous doctor come back for a series about adventures that they could have had? For example a series with Christopher Eccleston and Billie Piper of adventures that they could have had?
There is an independent production company called Big Finish, which has for years produced numerous full-cast audio dramas set in the Whoniverse, featuring old Doctors/companions etc (all with official BBC blessing, hence being allowed to use the actual "Doctor Who" name/brand). All the surviving Doctors from Tom Baker to Paul McGann have participated in these for years. They are retailed on CD and/or download, and some have been broadcast on the likes of BBC Radio 4 Extra.
It's a matter of personal opinion whether one wants to count such spin-off media as being "canonical" or not, but the narratives aim to fit in with (rather than contradict) the established TV timeline. The stories are set to fit in-between the TV stories from back-in-the-day, and the more aged appearance of the actors isn't an issue because it's audio (as long as their voices don't sound obviously much older).
I firmly believe that anything other than the "current" part of the Doctor's timeline is best kept to the like of Big Finish, and has no place on an outlet such as primetime BBC One. You could easily fill at least one entire television channel's schedule with all the side-steps and spin-offs stuff, but that doesn't mean that you should - because (like it or not) it's a niche interest, not a mass mainstream audience interest.