Regarding serving Huddersfield (which I would prioritise for London services, as it's one of the largest towns without a London service that does not have a major city nearby e.g. Bolton) , unless the Mirfield rebuild makes connections onto GC a lot easier, there are three possibilities of how to route the train:
1. via Sheffield and Barnsley - this would give a number of stations a London service (Meadowhall, Barnsley, Denby Dale and possibly more), but for that you would need to find a way to slot in an intercity train between the eventual 2tph all-stops services on the line. There may also not be as much demand as expected from some of those stations to London.
2. The existing GC route from London to Wakefield Kirkgate, then on to Huddersfield. Most likely the simplest route, could have it as a Bradford/Huddersfield joined GC service which splits/joins at Mirfield. Just having an improved Huddersfield-Mirfield service and an easy same-platform connection at Mirfield seems easier for this though.
3. Routing via the WCML, most likely post-HS2. Classic route to Stockport, then either Piccadilly and round the Ordsall Chord, or using the Stockport-Stalybridge line. If an OAO were to try this route, there would most likely also be stops at Stoke and along the Trent Valley, but running right through Manchester would likely not get approved due to significant revenue abstraction, so instead, only stopping at Reddish South or Stalybridge, then on to Huddersfield, could work.