Tennant recently announced he was leaving the science-fiction show after he wraps several specials set to air in 2009. He became the 10th actor to play the role of the Doctor in 2005.
The Daily Telegraph quoted him as saying his former "Doctor Who" sidekick Piper could theoretically take over the role when the series returns in 2010.
"Why not? It's one of those parts that any actor could bring something valid to, because it can be anything and it's a sort of blank canvas every time," he told the BBC. "The fact is that the difference is a virtue with each Doctor. It's not like you're casting Tarzan where you've got to have somebody who looks good in a loin cloth. It can be anything."
Tennant also emphasized that as of now, he didn't think a decision had been made regarding casting.
"I genuinely do not know who will take over. Last I heard, no one knew," Wales Online quoted him as saying.
The Telegraph said other actors whose names are being discussed for the role are Paterson Joseph, David Morrissey and James Nesbitt.