Comments from Doctor Who showrunner Steven Moffat seem to suggest that The Doctor might slip back into an old familiar face once Peter Capaldi leaves the role.
If Doctor Who did bring back Smith as a full-time Doctor, the reaction from fans would likely be mixed at best.
Instead of casting a new actor or actress to play The Doctor, the role might revert back to Matt Smith (who played the Eleventh Doctor before Capaldi took over the role).
There’s always a lot of speculation about what form The Doctor will take with each upcoming regeneration, since every time The Doctor’s form and personality changes.