A film is tricky business on every level, and a major risk. You just need to figure out whether the odds are reasonable when taking the plunge where those risks are concerned.
Assumed Knowledge
Some TV-show-to-movie projects work on the basis that the viewing audience is largely aware of the characters being portrayed in the film - that most people who pay to go and see the film are fans already. This can work...sometimes. There are other things to consider though - a large audience for a late-night Friday comedy series is differently defined to a large audience for a Saturday primetime sci-fi show. A large and devoted TV viewership needs to be able to convert into a reasonably large potential movie-going audience as well. In the case of Doctor Who, you need to be able to secure yourself a box office win that will earn back the costs of production - which are likely to be considerably higher than that of, say, The Inbetweener's.
A Show About Change
As others have mentioned, you have the issue of who to cast in your lead role. Doctor Who is such a timeless show - since its return in 2005 we haven't had a single series that hasn't made additions or changes to the title cast. In fact 2011's Series 6 (and soon Series 9 as well) is the only series that didn't lose anyone from the main cast. Who do you cast as a Doctor and companion so definitive that they get a role in the film? Do you stick with the ones that currently appear in the TV show? Do you cast wholly original actors, and if so how will they be canonised and remembered by the TV show? Paul McGann is undoubtedly popular as The Doctor, but his greatest success has been with Big Finish and The Night of the Doctor. Few would call his debut adventure a definitive movie experience. A show so rooted in the concept of change and timelessness would have to defy its own nature in order to make a movie work.
Premise
Another major question is what kind of event do you go for that could warrant a Doctor Who movie? The TV series already has rare levels of ambition - past, present, future, parallels, alternates and everything in between. We've seen warzones brought to the screen with what is comparatively stunning CGI, and though some ambitious ideas aren't quite done adequate justice (Journey to the Centre of the Tardis) you then have to wonder whether some things are best left to the imagination. What event could be so big that it warrants a film to tell it? Presumably it would be spectacular, and for lack of a better term 'game-changing'. The X-FIles: Fight the Future took this on board - dealing with a major storyline between Seasons 5 and 6 of the TV series, upping the budget a little and having a healthy dose of exposition on either end. But in that instance it worked because Tbe X-Files doesn't have the issues Doctor Who has - it's not as timeless, it's characters are iconic and played by actors who own their roles exclusively. Doctor Who is far more tangible in principle, and you can up the budget all you want for a feature film, but can a feature film depict something unique that the TV series already can't?
Format Alienation
Sometimes you also get the issue of a TV series losing its way once it takes to another platform or format. Buffy the Vampire Slayer is the best example I can think of - running for seven successful TV seasons, and then continued in a supposedly canonical comic series. Comics are apparently wonderful things, able to do what the TV series can't - it can bring back long-gone characters who were unavailable for return to TV because of actor commitments, and it can depict concepts on a grand scale that the TV series couldn't possibly afford to do. It sounds wonderful in principle, but there seems to be no restraint where this sort of thing is concerned. You can suddently depict giant battles, and turn your supporting characters into a centaur (no joke, they did that to a character in Buffy's comics) and bring back that character played by a high-profile actor you could never have gotten a second time on TV... but by doing so the concept loses the heart of what its depicting.
If you take Doctor Who and leave nothing to the imagination, take it to the big screen and depict the Time War in full epic 4K it might look great, but the charm is gone. The power to imagine is gone. Doctor Who can pride itself as a TV series that feels massive - it has a fully believable universe filled with all matter of life and bold characters. It hasn't ever needed flashy, movie-scale budgets to impress. There's a certain prestige and excitement at the show being so successful that such a concept is considered, but it would be very easy for a movie to alienate itself from its original product - for a Doctor Who Movie to not feel like Doctor Who.
BBC Politics
This one is more technicality than anything, but there are various hurdles in the way. The BBC can't be seen to be making a profit from the show it produces from the TV License. It could produce the film, and BBC Worldwide could foot the bill, but TV show viewers couldn't be required to watch the film to gain understanding of the TV show (despite the fact every fan would watch both). As such, the film under current conditions would likely have to standalone. Could it be canonical? Would they need to rest the TV show in order to fit in a movie? There are many things to consider even before you reach the point of drafting a premise.
Me, personally, am indifferent. A movie would be awesome, and unless they put David Tennant in the lead role (thankfully an implausiblility) I'd be seeing it the week it was released. But I'm also sceptical about it... I don't know if a film would work, I have doubts I'd enjoy a film as I do the TV series, and I hold the opinion that I'd rather see the TV series run undisrupted than having to cater to a film.
Not least because 12 is and can continue to be more than grumpy.
