On another thread recently, I highlighted how a journey from Birmingham to Cambridge will become quicker going via London once HS2 phase 1. The same could also be said for Birmingham to Peterborough which currently takes 1hr 50m on the direct train, but post-HS2 it could potentially only take 1hr 40m if the connection times match up (45 mins Curzon St - Euston, 10 mins walk to Kings Cross, 45 mins KGX to Peterborough on LNER). With this in mind, I think it would be worth discussing whether there is still a need for the Birmingham - Stansted service to continue in its current form in the long term.
One option could be to split the service at Leicester, since from there it will still be quicker to use the direct service to Cambridge at least, but with this you lose the direct connectivity between Birmingham and Melton/Oakham/Stamford. Splitting at Peterborough is another option, which could allow for 2tph between Leicester and Peterborough, but at the expense of direct Leicester - Cambridge connectivity. Alternatively, you could divert it to elsewhere in East Anglia (Norwich, Ipswich, Colchester) and have the orphaned portion to Stansted form part of another service. I’ll leave it up to you to discuss which option would be best.
One option could be to split the service at Leicester, since from there it will still be quicker to use the direct service to Cambridge at least, but with this you lose the direct connectivity between Birmingham and Melton/Oakham/Stamford. Splitting at Peterborough is another option, which could allow for 2tph between Leicester and Peterborough, but at the expense of direct Leicester - Cambridge connectivity. Alternatively, you could divert it to elsewhere in East Anglia (Norwich, Ipswich, Colchester) and have the orphaned portion to Stansted form part of another service. I’ll leave it up to you to discuss which option would be best.