There has been a lot of talk recently about Matt Smith possibly making a return to Doctor Who after current Doctor Peter Capaldi leaves the show. The driving force behind this discussion is comments made by current showrunner Steven Moffat, concerning how much Smith would like to come back to the show. Of course, this is all just talk for the moment; there has been no real indication that the Eleventh Doctor will be back for a revival tour. That doesn't mean that Smith won't return, but it also doesn't mean that he's about to sign on the dotted line.
If Smith were to come back to the show, there are three prevailing theories as to how it might happen. The most obvious one is that he might show up for a multi-Doctor episode before Moffat's run as showrunner is complete, meaning that he'd make a guest appearance sometime in season 10. This is probably the most likely option, since Moffat is known to reuse characters he's created, and Smith took over as The Doctor when Moffat took the reigns on the show.
The second theory holds that Smith might take over as The Doctor again when Capaldi leaves the show. This would be a first onscreen, though it's not without some possible foreshadowing: During the 50th anniversary episode "Day of the Doctor", Smith met a museum curator played by former Doctor Tom Baker (who may or may not have been a future version of The Doctor) who suggested that The Doctor might find himself revisiting a few faces... "but only the old favorites."
The third theory is perhaps the most interesting. It states that Capaldi's Doctor might encounter some problem with his regeneration, and temporarily revert to Smith's Doctor (and possibly other previous Doctors such as David Tennant or John Hurt) before finally assuming a new form. This would circumvent some of the problems that people might have with the Doctor appearing to take a step backward in his regenerations, while letting several former Doctors have one more round with the sonic screwdriver.
Let's set the possibility of a multi-Doctor episode aside for the moment, since it's both the most obvious solution and something that's been done several times on the show. Doctor Who tries to bring something new to each incarnation of The Doctor, and bringing Smith back in the starring role would definitely be new territory. That's why it might be just what the show needs as it transitions from Moffat to incoming showrunner Chris Chibnall at the end of season 10.
Transitioning from one Doctor to another can be hit or miss, and transitioning at the same time as a showrunner change makes it even more difficult. That's part of the reason that the BBC has asked Peter Capaldi to stay on for another year past season 10. Smith managed the dual transition once before, becoming a new Doctor at the same time as the shift from Russell T. Davies to Moffat, and he owned the role. Bringing him in to help with the transition from Moffat to Chibnall would make things go a lot smoother, since he's not only familiar to audiences but also to the unique stresses of the role during a time of transition as well.
The Doctor is a complex character, and Smith's Doctor appeared to shrug off most of that complexity when he hit the scene with his awkward eating habits and need to wear "cool" things like bow ties and a fez. He was an adorably dorky Doctor after Christopher Eccleston's jaded survivor and Tennant's quirky romantic. This was largely an act, of course, as Smith showed off his range when bits of darkness slipped through the cracks in his carefree demeanor. The character suffered later in his run as a result of inconsistent writing and a largely "monster of the week" approach to the show in advance of "Day of the Doctor," but when Smith's Doctor was at his best he was fantastic.
Perhaps the most appealing thing about Smith returning to the role is that the unique nature of his return wouldn't be lost on him. He would have the same face and perhaps the same mannerisms, but he would also toy with questions of why he was back and how much longer he could go on. Remember that Smith's Doctor is the one who died of old age; between that and memories of Capaldi taking "the long way around" he could very well be weary and wanting to reach an end of his journey. This could pair especially well with the Doctor's new companion Bill, who might be surprised to find such a young-looking Doctor feeling so old.
If Smith's Doctor did return, what would be his purpose? That would be easy if he was a "temporary" Doctor, the result of a regeneration gone wrong that's regressing The Doctor back through previous lives. If he were the "actual" Doctor, though, then it would need to have a better reason behind it. Similar to how Capaldi's Doctor took the face of Caecilius from "The Fires of Pompeii" (which was Capaldi's first role on Doctor Who), The Doctor might return to Smith because of some unfinished business from the days of the Eleventh Doctor.
This could also tie in with the Time Lords, who returned from extinction thanks to Smith's Doctor (with assistance from the other twelve versions of him, including Capaldi) in "Day of the Doctor". While some of The Doctor's frustration with the Time Lords may be explored in season 10, it's possible that season 11 could see him taking more of antagonistic role against his people as a result of continued meddling in his life (as seen toward the end of season 9). If that's the direction that the show was taking, they might even be responsible for the next regeneration.
It remains to be seen what the future of Doctor Who is, since we've still got five months until the next Christmas special and an even longer wait until the next season begins. It's possible that we'll receive definitive word in that time on Capaldi's decision to stay on another season, or leave when Moffat does. If he does decide to leave, then we should meet the next Doctor sometime in 2017.
Will it be Matt Smith when the reveal is finally made? Probably not, unless the BBC goes with the "regression" theory and he serves as a temporary Doctor. We might see him in a multi-Doctor episode, though. Then again, there's always the chance that Doctor Who will take the big risk and bring him back for another run. Some wouldn't like it, of course, but it would have the potential to be amazing.
Doctor Who returns in December for its Christmas special, and season 10 begins some time in 2017.