I think the GWR Large Prairies should pull GWR suburban trains. The Railworks B Set is available, but the B Sets only worked in 2-Coach formations on branch lines. And the 5100, 5101 and 6100 classes were built for suburban and local passenger services. So I was thinking the locomotives could pull coaches like these:
Most useful would be the Hawksworth suburban non-corridors (diagraams 83,132,166,167) or the earlier Collett non-corridors (diagram 109,117,131).
Why not ask Matrix Trains if they are likely to produce these coaches? They already make GWR Coaches - https://store.steampowered.com/app/..._Churchward_Panelled_Toplights_Pack_01_AddOn/