the Conwy Valley goes unmarketed with an infrequent, poorly-coordinated timetable and rotting, filthy 150s chugging their way down the branch
The October 2016 issue of Modern Railways has an article on the Conwy Valley Line. Interestingly, in the context of your post, it apparently has been given an increased promotional budget and, apparently, a new line logo.
It's hard to know what will happen to the 175s, since if they do decide to order coaching stock for the long distance services, it'll make the fleet largely redundant - completely redundant if W&B decide to go for the 185s that are soon coming off lease to work the North Wales inter-urban services.
The problem with the 175s, as I see it, is the main regional expresss routes they work are getting rather too busy for 2-car units and even the 3-car sets struggle in places. You could just bang them together to make longer trains, but since they don't have UEGs that's not ideal (if I was writing the franchise agreement, I would want a ticket-checking, travel-advise-giving member of staff in both units if they did that, and would rule out in-service splitting of units without UEGs). The new Conwy Valley Line logo (page 74 of the October 2016 Modern Railways) has a train on it, and that train; wait for it; is a 175. Could be wishful thinking on the part of whoever designed it of course, but apart from not being able to make use of the unit's 100mph top speed I think it actually makes a lot of sense. The 175/0 fleet would be a reasonable choice for the Conwy Valley and Heart Of Wales lines I think (the other good choice would be 156s, but only 14 of those are up for grabs and if Wales got them at least half would be needed for Swansea-Pembrokeshire services; more in the summer). Give the tourists a nice comfortable train, not an inner-suburban unit (which 150s essentially are).
Going back to the 185s; if the 3-car 175s are inadequate then a 185 will be also, and the 175 is a higher standard of train. Plus, as far as I've been able to dig up online, the first 185s don't come off lease until Novemeber 2019, so how many will be available by the all-important 2020 PRM deadline is unclear.
I don't think Holyhead-Cardiff should be as frequent as it is, let alone become even more frequent, but realistically I can't see the service level being reduced in the next franchise. Other services will have to work around it.
Assuming Holyhead-Cardiff remains bi-hourly and express, the suggested all-stops Holyhead-Llandudno could be bi-hourly, to make an hourly frequency between Holyhead and Llandudno Junction, with a couple of fast extras to each of Manchester and Birmingham via Stafford, along with the London trains.
The current 2-hourly Holyhead-Cardiff is NOT express; only 'Gerald' is and even that has been slowed down since originally introduced (and always did use a stopping path northbound out of Cardiff). Although I propose cutting the frequency to 3-per-day, I do think they should all be express, going back to the original southbound calling pattern of 'Gerald' (except calling at Wrexham vice Crewe and allowing the franchisee the choice of whether or not to stop at Hereford in addition) and with hot food available. Thus, it could be spun as an improvement as the remaining 3 trains per day would be of a higher standard than most of the present services.
Then hourly Bangor-Manchester semi-fast, with a couple originating at Holyhead as I say; the Holyhead ones could be portion workings, with the train splitting at Chester and the other half going to Birmingham via Stafford.
That sounds a little backwards to me; since if you split at Chester while heading east you have shorter trains at the busier (England) end of the route. If we had enough suitable trains (the only design I can think of is the class 158/159 and all ATW's 158s are needed on the Cambrian-Birmingham-Wrexham/Chester curcuit) I would do it the opposite way round, with a pair of units coming out of Manchester (or Liverpool, or Birmingham) and splitting with portions for Llandudno and Holyhead, Llandudno and Caernarfon or Holyhead and Caernarfon.