The LSWR's line to Plymouth diverged at Coleford Junction on the Barnstaple line, and headed to Okehampton, and then to Tavistock, joining the Gunnislake branch at Bere Alston. It would make sense to reopen this route because due to global warming causing rising sea levels, the line to Dawlish may end up under water.
However, when the GWML is finally electrified, it would make sense to electrify the Valleys lines around Cardiff, the Severn Beach line in Bristol, the line to Western-Super-Mare (or even Bristol-Exeter in its entirety so Cardiff-Taunton services can be EMU operated), the lines around Exeter (the Tarka line, the Exmouth branch, the Riviera line, and even up the West of England line to Axminster with a passing loop to allow a half-hourly local service there), and possibly the Tamar Valley Line. Although these lines would not be electrified immediately, it would be a disappointment to get off the shiny new electric train at, say, Exeter St Davids, and be confronted with a 150 for your onward journey (although by the time the GWML is electrified the 150s should have been replaced by more modern units following successive rolling stock cascades).
Electrification of the aforementioned lines should be in conjunction with infrastructure and capacity upgrades to encourage a greater modal shift from road to rail. For instance, the Tamar Valley line could be redoubled from St Budeaux Victoria Road to Bere Alston to allow a more frequent service; the proposed extension to Tavistock to serve a new housing development would attract more passengers. Rolling stock would be cascaded 317s or 321s to save money, until new EMUs are ordered.
Trouble is, none of the above will happen because it's too expensive, and it's not likely to happen unless a government is elected which regards public transport as a social necessity rather than a commercial enterprise (and no, I don't mean Labour). I wouldn't be surprised if the Tories cancel electrification of the GWML if it doesn't meet their value-for-money test.