1. Too much time has passed for the classic actors. I would LOVE for Tom Baker to return but I doubt the BBC has the budget or desire to CGI de-age him.
2. Eccleston does not want to come back.
3. Tennant was very popular and had great stories.
Also his portrayal of Killgrave on Jessica Jones tells me that if they cast him as a more sinister version of the Doctor or even as THE MASTER that it would be a hit.
They've established that revisiting old favorite faces is possible when Doc 11 meets the Curator, and Doc Ten did abort a regeneration process to heal himself from a Dalek blast but remain as himself. So regenerating back into the appearance of Doc 10 but with an altered personality would be interesting
With the right directors, producers, writers.....
ALL that they need to do would be to bring back thre david tennet clone, the full human one!