.....elements that are outdated and no longer fit for purpose.....
I guess this is not exactly what you had in mind but I think the lack of
OHLE▸ between Thingley Junction & Bath (and obviously onward to
BRI» ) is something that should be rectified ASAP.
Also change the track layout / signalling at Thingley Junction so that trains running NE off the Melksham branch don't need to run 'wrong line' at all.
Definitely a new station, with platform loops, at Royal Wootton Bassett (population c.15,000).