"Stagecoach 12139 on Route 15, Charing Cross" by Au Morandarte - Flickr: Stagecoach 12139 on Route 15, Charing Cross. Licensed under CC BY-SA 2.0 via Wikimedia Commons.
Transport for London have confirmed that Route 15 will be converting to New Routemaster buses; in fact they already have hybrid buses in service.
Their press release
Transport for London (TfL) has today announced that the next bus routes to be served entirely by New Routemaster buses will be the 55 and the 15.
A phased conversion of route 55, which is operated by Stagecoach, is underway and will be completed by the end of the month. The route runs between Oxford Circus and Leyton via Tottenham Court Road Station, Old Street, Shoreditch High Street, Cambridge Heath, Hackney Central and Clapton.
During peak hours, 34 of these state of the art buses will be in passenger service to carry the 27,000 people who travel on the route each week day.
Route N55, which runs from Oxford Circus to Woodford Wells via Bloomsbury, Clerkenwell, Old Street, Shoreditch High Street, Cambridge Heath, Hackney Central, Clapton, Leyton (Bakers Arms), Wanstead, South Woodford and Woodford Green is also being converted to New Routemaster buses.
Route 15 will begin its phased conversion at the beginning of February and will be entirely with New Routemaster buses by the end of the month. The route, which is also operated by Stagecoach, runs from Trafalgar Square to Blackwall DLR via Aldwych, St Paul's, Mansion House, Cannon St, Monument, Tower Hill, Aldgate, Aldgate East and Limehouse DLR.