For the MML rolling stock, although the Meridians can stay for at least another 20ish years, the HSTs need to be replaced within the next 5 years, it is completely unacceptable for 40+ year old trains to be running on a mainline, especially as such trains have never received a proper refurbishment, and still have exactly the same seats that they were built with.
Personally I think Network Rail should focus on continuing the Sheffield branch, allowing 2 trains per hour to turn electric, freeing up enough stock to completely displace the HSTs, while I would also stop the direct service to Corby, to be replaced by a 153 shuttle to and from Kettering, with the direct service instead diverting to Leicester to also allow this to be electrified.
I personally would go for Sheffield, over Nottingham because this will allow trains to access current depots.