Oof, no no no. Former players rarely make great coaches. They often make serviceable coaches, but rarely great ones. The skills required to be a great player are largely totally distinct from the skills required to be a great coach. Either Manu or TD would make a good assistant coach, but I think a head coaching stint would sully either reputation.
Pop's successor will depend in large part on when he decides to hang them up. If he hangs them up in the next 2-3 years with TD as he's threatened, I'm hoping the Spurs bring over Messina.