What I find interesting about Capaldi's future as the Twelfth Doctor is what bearing this will have on the series after his incarnation is complete, and if the answer of whether or not in this post-Time War world if the Doctor is bound by the thirteen regenerations still or he is able to go on and on.
I mention this because it stands to safely assume that of the new series Doctors thus far, Tennant has proven to be the most popular and probably the best between the trio of himself, Eccleston, and Smith. He is also the longest running of the new series Doctors, running from 2005-2009, and then in the specials that ran until 2010, concluding the story of the Tenth Doctor. Tennant has also beaten Tom Baker as the most popular Doctor of all on two occasions in Doctor Who Magazine, and has also been voted as the UK's favorite Doctor just this past year.
While time will tell in regards to the longevity of Matt Smith's Eleventh Doctor, and Christopher Eccleston's Ninth's legacy in danger of being forgotten entirely, Tennant will be seen as an important contributor to the series, even as he himself ages and grows older.
The Tenth Doctor will be immortal