Cardiff to Birmingham wouldn’t have much impact as the majority of passengers only travel between there. It would also make the trolly system easier and the capacity management would become a lot easier.
Weston and Gloucester calls wouldn’t make them happy but Gloucester’s MP▸ (Member of Parliament) has tried lots of options to get mainline services to call at Gloucester, so it would have to be considered. Weston is a bit too far out (in my opinion) to expect express services to call at but again would be useful. As a passenger myself it isn’t to annoying to go to Cheltenham to connect, however some people don’t like changing and peak GWR▸ (Great Western Railway) services to Bristol are often packed.