If we did that, we would end up with 158s on routes such as Bristol-Weymouth and Nottingham-Skegness, which they are not really suitable for. I wouldn't mind seeing them take over Leeds/Bradford-Carlisle (although I would prefer to see Nottingham-Carlisle expresses) but we already have them on the Far North, where 90 mph speed is unnecessary and 156s would be better (IMO).
However, to get that, what we need is more electrification in the West Riding, West Midlands and Scottish Central Belt, as well as an extension of the Thames Valley scheme to Chiltern, which would cut down on the need for DMUs on suburban routes and allow them to cascade into the country. Then, we build the 'Son of Sprinter', possibly based on the 172 with a few modifications to reduce emissions, lighten weight and improve ride on rough track. It needs a gangway, and can appear in 1-car, 2-car or 3-car versions. That gives us one class of local DMU, working St Erth-St Ives, Norwich-Sheringham, Peterborough-Lincoln-Doncaster, Leeds/Bradford-Morecambe, Newcastle-Carlisle and Inverness-Wick (plus a lot in-between).
