Thranduil also happens to be Legolas' father, which sheds some light on how Orlando Bloom's character will be reintroduced to the franchise. Tennant also recently confirmed himself for a new take on Much Ado About Nothing, so it is not clear how he plans to fit both films into his upcoming schedule.
At this time, David Tennant's casting in The Hobbit has not been officially confirmed. David was originally being considered for the role of Bilbo Baggins, a part which has gone to Martin Freeman. Sylvester McCoy is also rumored to have been offered the role of Radagast the Brown, but there is no confirmation that he has accepted the role at this time either.
The Hobbit: An Unexpected Journey was released December 14th, 2012 and stars Ian McKellen, Martin Freeman, Richard Armitage, Ken Stott, Graham McTavish, William Kircher, James Nesbitt, Stephen Hunter. The film is directed by Peter Jackson.