The elephant in the room that nobody mentions is electrification or lack of it.
As others have said until there are new spare DMUs▸ such services are a bit of pipedream, and the likelihood of their being spare units even with new build which are basically to relieve existing overcrowding is unlikely expecialy if the 14Xs are withdrawn or more likely become so unreliable nobody will let them out of the depot.
The availability of dual voltage electric rolling stock will increase markedly as Thameslink proceeds towards its final condition, with well over 1000 brand new vehicles in 8 or 12 car fixed formation. So being an optimist, the infill electrification will be needed so as to avoid units like 319s being scrapped 10-15 years early. Another key change is that Thameslink KO2 doesn't really introduce new routes, it absorbs existing Southern, Southeastern, and
FCC▸ /GN routes into Thameslink, and will displace hundreds of 'mid-life' units from their existing electrified routes.
So I think Basingstoke - Salisbury - Exeter and Salisbury - Southampton/Eastleigh will be obvious solutions to soak up rolling stock, AC or Dc doesn't really matter as much of the stock can be made dual voltage even if not already. The
SWT▸ 158s and 159s, and then the SN Turbostars should be snapped up elsewhere, such as Norwich - Liverpool, especially if run as 6 or 9 car sets like Waterloo Salisbury etc...
Paul