I agree with DaveNewcastle's post; local stations on IC routes tend to suffer for the benefit of expresses except where the line is quadrupled.
This can be seen on the GWML too: there's an abundance of local stations (and services) between Paddington and Didcot, where it's 4-track, but little west of there, where the line goes down to 2-track.
Just looking locally, between Didcot and Swindon there could be stations (and were in the past) at Wantage (Road/Parkway), Steventon, Shrivenham and possibly Milton Park (not that I'm biased there). Wooton Basset is an obvious case, fairly big in itself and well placed for a parkway station. Chalford and Brimscombe near Stroud would be nice too.
As an example of such campaigns working, Cam and Dursley station was opened in 1994, and is only about 5 miles from Stonehouse station (although they are on different lines).