At the moment Aggers very much leads the TMS coverage, he has the first stint of the day and will often commentate on the denouement or the close of the day so as to run straight into the interviews and round up of the day for the podcast etc. Aggers is also often called into other BBC radio to comment on cricketing stories or in the morning to preview a days play if a test is well poised or there is a particular milestone about to be achieved.
I suspect he will step back into a more general commentator role with the new correspondent having the interviews, online written column, and the appearances into other radio programmes such as R5 or The Today Programme when previewing play or responding to cricketing stories.
I wonder who will take on the role, whether they will want a 'name' who has had a significant international career or if a more journeyman cricketer like Dan Norcross who has been commentating for a few years will be a better fit, Kevin Howells who has led the county cricket coverage would be another established option. There is also the option of some of the ladies who have transitioned from playing to commentating who could be an interesting possibility.
