In the shorter term, I suggest that some could be usefully put on new semi-fast services on an electrified MML. For example, London St. Pancras to Corby / Derby / Nottingham, calling at St. Albans, Luton Airport, Luton, Bedford, Wellingborough, Kettering - splitting there with one portion for Corby, the other portion for Market Harborough, Leicester, Loughborough, East Midlands Parkway, then Beeston & Nottingham, or Long Eaton & Derby. (A sort of MML equivalent of the LM services on WCML)
This would allow the new "express" stock to improve journey times to the larger cities & towns, calling only at Leicester, then either Nottingham or Derby / Sheffield.
Others may be stored for a while, pending possible use when the next batch of electrification schemes is started (2025 - 2035)
That might include some of:
Birmingham Snow Hill area services;
Calder Valley services (including Hebden Bridge - Preston);
Liverpool - Warrington Central - Manchester.
Manchester - Atherton - Wigan - Southport.
Leeds - Harrogate / Castleford / Barnsley / Sheffield routes.
Manchester - Marple line (assuming not grabbed by Metrolink)
Having some older emus available would be useful in reducing initial costs, with an assumption that newer units would be obtained in a few years.