Andy Burnham's decision to run as Mayor of Manchester (http://www.theguardian.com/uk-news/2016/may/18/andy-burnham-to-stand-for-mayor-of-greater-manchester) is an interesting move. Like Sadiq Khan and his successful election as London Mayor, this would give Burnham a very high profile position in British politics and real executive experience at a time when Labour have been out of power. When any party has been out of power for an extended time it is easy to wave the "no experience" card at them, but if you can say, "well we have been running London and Manchester", that takes a lot of the sting out of that one.
I think it may also allow the likes of Burnham and Khan to cast a distinctive position away from the constraints of Westminster and 5 minute news cycles. You may be high profile but you are dealing with more tangible matters such as bus fares and policing. The more devolution that happens, the more that local politics becomes a sensible route into politics. Perhaps our future leaders will no longer come from such a narrow base?
Could he use it as a springboard to Labour leadership ahead of 2020 if Corbyn does badly, say in the 2018 council elections (London Boroughs, Metropolitan Boroughs and many second tier cities like Cambridge and Norwich)? Possibly, but he will be 1 year behind Khan and will be able to show little progress in such a short time.