Doctor Who has a history of writing off characters in the most heartbreaking ways imaginable. Companions rarely seem to get happily-ever-afters when they leave the TARDIS for the last time, and each regeneration of the Doctor usually comes with a few tears. Luckily, the heartbreak is not always permanent thanks to Who's love of using timey-wimey shenanigans to bring back departed characters. Now, it looks like Matt Smith would be more than willing to reprise his role as the Eleventh Doctor despite the fact that Doctor Who is currently on the Twelfth. Here's what Smith has to say about the possibility of a return to Doctor Who:
I really want to...But then Peter [Capaldi] would be like, 'Hey - it's mine!'
Matt Smith revealed that he'd be up for another go at Doctor Who during a Wizard World panel (via The Doctor Who Companion) in response to a fan question. Smith departed Who in 2013 after three seasons and two specials, one of which was the 50th anniversary episode that saw the return of David Tennant's Tenth Doctor. In fact, the 50th anniversary episode was the second-to-last for Smith; he was succeeded by Peter Capaldi as the Twelfth Doctor in the next. Matt Smith seemed confident in his decision to leave the show at the time; perhaps the years since have changed his perspective.
There's a big difference between Matt Smith mentioning at a convention that he'd like to come back and Matt Smith actually coming back. Fortunately for fans of Eleven, the Mirror noted that Doctor Who showrunner Steven Moffat has said that Smith hasn't been shy about expressing that he misses the show and wishes he hadn't left. Moffat himself doesn't have much time left with Doctor Who, so there's no way to know if replacement showrunner Chris Chibnall will hear anything encouraging from Smith.
Doctor Who has brought back characters who were supposed to be gone for good plenty of times over the 50+ years that it's been on the air. Ever since the revival in 2005, characters ranging from Rose Tyler to the Tenth Doctor to Clara Oswald have made impossible returns. Still, a past Doctor appearing for something other than a milestone episode doesn't happen every day, and there are a couple of ways that it could work for Eleven.
The most obvious way for Matt Smith to pop up on Who again as Eleven would be for Twelve to cross paths with his past incarnation and share an adventure. It worked when Eleven met up with Ten and the War Doctor in the 50th anniversary special; it could definitely work again for Eleven and Twelve.
Given that there are rumors that Peter Capaldi is planning on leaving Doctor Who after the upcoming Season 10, however, perhaps Who could get a bit more timey-wimey and bring Eleven back to the airwaves in a less traditional way. The most wild would have to be if Twelve somehow regenerated into Eleven, which would certainly break a lot of rules of Doctor Who time travel. Still, a lot of traditional Who time travel rules have been broken during Steven Moffat's tenure on the series. The Doctor going from Eleven to Twelve to Eleven isn't totally out of the realm of possibility after all the precedents set by Moffat. Of course, Matt Smith could just come back as an Eleven from a time before he regenerated into Twelve and star in a solo special or two that just happens to be set earlier in the Doctor's timeline.
There's no saying at this point if Matt Smith will reprise Eleven again anytime in the near future. At the very least, it's nice to know that he's open to coming back at some point. Doctor Who is likely to have reunion specials again, and Smith's enthusiasm could mean that Eleven fans won't be in for as much of a disappointment as faced Nine fans when Christopher Eccleston declined to participate in the 50th anniversary. Only time will tell.