I think it would be cool if they left Tennant on a cliff hanger and did a whole series with McGann.
(I like Tennant, but I am ready for new blood).
Personally I'd let Tennant fall off the cliff instead of leaving him hanging there
Maybe under the new regime the writing and directing could improve to the point where Tennant's major weakness ( his acting in any situation where he's not required to be a smartmouth ) isn't so apparent but he's gone way past the point where I care about his version of the Doctor and I would sooner see someone else in the role.
Aside from Johnny Depp who would likely cost the BBC too much money I have no idea who could replace him but there's got to be someone who can bring some depth to the character and have that slightly odd "not of this Earth" quality that is nowhere to be seen in his 10th incarnation.