[user104658]

It could be interesting because he'd have to play it a little differently - when he left, the doctor was relatively “young" compared to now. In terms of the doctor's own timeline, the Smith and Capaldi eras were LONG in terms of the doctor's actual age. On Tennant's exit he was canonically 907 earth years old. When Smith and Tennant meet, Smiths doctor is over 1200 years old (he's done a lot of "off screen" travelling, spent time travelling with River Song, had various companions we've never really seen, implied in several episodes).

He then spends 900 years at Trenzalore, and Capaldi's Doctor is stated to be over 2000 years old... And then during that era, the doctor is implied to spend large amounts of time travelling alone in the time between picking up/dropping off Clara.

Its strongly suggested by the end of the Capaldi era that he's at least 2500 years old but actually no longer knows the exact number (and it's not like you can go by date of birth). Age doesn't really come into it after the first Capaldi season. Just "... old".

ANYWAY, the point is that when Tennant was last the doctor he was a spring chicken of about 900 (he's supposed to have stolen the TARDIS aged around 250) and now the doctor is at least 2500-3000 years old. Loads of scope for him playing the same personality, but with that extra age and experience, which would of course tie in with Tennant himself being physically older. I think it would be a very interesting thing to explore.

If any of it is true, of course.

Of course in terms of the show/production - again if its true - the skeptic in me would say its likely because they want to make the doctor male again and they'll get far less backlash if it's bringing back an old, popular version of the character than they would immediately recasting a new male actor. Plausible deniability etc. "Oh we would have gone with a woman again but we had this opportunity for Tennant to come back to the role" etc