easyJet maybe? The EZ/MAN rumour has been flying round for ages. Trying MAN from an existing base could test the waters?
bmiBaby- with 4 aircraft becoming available (2x new, 2xMME) and 3 going to BHX, MAD could be part of an expansion from MAN
Jet2- They already have Spain covered from MAN, so Madrid would be a logical choice. They would be my favourite to serve the city.
Ryanair- if MAD does ever become a base, as was rumoured some time ago, then MAN could benefit from a route- this is highly unlikely I guess, but possible.
Vueling- entering the UK market must be highly lucrative, but very competitive, so having gained brand recognition in Spain they might be ina good position to give the UK a go?
It really could be anyone!