I think its been mentioned but electrification of the Thames Valley branch lines, especially the Windsor branch. You've got turbos running around from West Ealing, Slough, Maidenhead & Twyford which would much easier be 387 4-cars to have a unified fleet and allow turbos to be cascaded further west, and improve loadings as required. The extra cost must not have been that prohibitive for the overall gain.
The problem with most of those is platform lengths - the Greenford branch has definitely got short platforms and Maidenhead - Marlow was discussed on another thread not so long ago.
Even 3 car EMUs (which would be sub-fleet) would be a struggle on some of those - it's why they're run using 2 car DMUs.