I think it would be interesting to see the Snow Hill Line in tsw, the diesel rolling stock and the different selection of stations means it would be a completely different experience to the Cross City line, the portion I am suggesting is the section between Snow Hill and Kidderminster. This takes around 35-40 mins, it could also go to Dorridge in the other direction as well as that would take the train through moor street and past the Bullring.
Kidderminster to Stratford would seem to also be a logical option, but this would "orphan" a number of runs from Snow Hill and moor Street that run down the mainline to London Runs would be 80 mins end to end, 31 stations over 44½ miles and include all the assets noted in the OP
