Interesting topic.
I always feel besmirched that XC get to go through Gloucester but refuse to stop here. Back in my youth I used to love to go trainspotting upon the tump, now Metz Way and watch as a 47 would pull into the station and either run around or be replaced by another waiting in the siding by Horton Road gates. Stopping was never an issue back then.
The thing with XC and delays is nonsense. Isn't it a fact that they could bypass Reading station and call at Reading West? Does anyone travel on XC services from Edinburgh to Bristol? Stopping here wouldn't inconvenience many, they don't mind inconveniencing Gloucester people trying to catch their trains though. I know they're never going to stop here so it would be a better solution to build a new mainline station at Waterwells to serve the expanding suburbs in south Gloucester and hopefully satisfy those in Stonehouse (and Stroud) who want a station on the Bristol line.
I read an interesting point about pathing on the GWR mainline at Yate, but if it was possible then XC could stop at Waterwells and Worcesterhire Parkway and bypass Cheltenham. Lansdown is in a dreadful location with inadequate parking, no use to anyone.
Interesting to read between the lines though and assume that XC don't actually want any more passengers!
Personally, your post proves the need to move Gloucester station from it's current location.
It could easily be on the North side of the triangle and cover both trains South and West, as, IIRC, no train comes up from the Newport Gloucester line and then goes South (other than diversions or freight which don't stop at GCR).
Admittedly this would add some distance to the city centre which is the major concern.
It's also worth remembering XC doesn't really chose their paths, they're still, to my knowledge, (and unfortunately I can't think of another analogy, happy to edit) the disliked step child who has to fit around others. Therefore, they need to fit around the likes of bristol and Birmingham, reducing the chance to stop at GCR.
Personally, some sort of automatic rapid transit from by Morrisons (where I'd put the station) would be best, but the finances may be an issue.