I've been trying to come up with ideas about the suitability of stock on both Cambrian lines that the two hourly service to Pwllheli and Aberystwyth still runs, but it could work as an express service with class 158 stock, perhaps calling at Welshpool, Newtown, Caersws, Machynlleth, Dovey Junction, Aberdovey, Tywyn, Fairbourne, Barmouth, Dyffryn Ardudwy (?), Harlech, Penrhyndeudraeth, Porthmadog, Criccieth (?) and Pwllheli and then a one carriage 153 (once fitted with the right equipment) stopping service (retaining the request stops as they currently are) could operate from Machynlleth to Pwllheli once every 2 or 3 hours. I don't know how this would actually work as I rarely travel above Harlech but I know there are passing points at Tywyn, Barmouth, Harlech etc.
This would also allow passengers wanting to travel to a request stop from Shrewsbury or beyond to potentially take the service to Machynlleth and then change. Aberystwyth would get a hourly service and Pwllheli will get a 2 hourly express service, and then the stopper.
I think the problem is that as most of the Cambrian coast is single tracked and a small delay could easily lead to massive delays and cancellations, and also, 2 or 3 153s would also be needed. Also, it might see less people using the already small stations/request stops, which wouldn't be a good thing IMO.