His nomination for the next big project was ... Didcot grade separation. His background is is track layouts, which may be a bit of a bias. However, I suspect his choice was based on where the next dominant capacity limit on the line is, as that's what justifies the big bucks needed to give him a really big project.
That makes a lot of sense, and confirms my gut-feeling as well. I would suggest, however, that although it is a stand-alone project, it really only makes sense in conjunction with capacity increases in the Oxford station area; it being silly to increase capacity in one place if a few miles down the track there is still a bottle-neck which limits the use one can make of the new junction.
As has been noted in other places, the need for grade separation increases (i.e., the incidence of conflict increases) more than linearly with increasing numbers of trains using each branch. After the Reading area on the
GW▸ it is clear that, on train numbers alone, Didcot is the next in line. After that then I would think that Wootton Bassett and Westerleigh Junctions would be good candidates and possibly something between Filton and the Severn Tunnel to separate and join the Bristol and London flows.
I don't think grade separation at Swindon could be justified even with a train on the Kemble route every half-hour as trains joining and leaving the main line could be held in the station until the route is clear. The other candidate that is sometimes mentioned would be grade separation at Southcote Junction just south of Reading where the Reading - Newbury - Taunton line separates from the Reading - Basingstoke line. Although it would be a 'nice to have' I can't see anyone being able to justify it at the moment as Reading will have had just shy of ^1 billion spent on it over the past few years and other areas will complain if yet more is spent in the same area. In any event I think the new layout will have to shake down before anyone will be able to justify more expenditure. But if the number of container trains to and from Southampton continues to increase and the density of traffic on the Newbury route also increases in the next few years then grade separation here will also have to be investigated.
But first - Didcot...and, I hope, within 15 years!