Doncaster, probably. I don't think there's a huge, untapped market for people wanting to visit Knottingley, Pontefract or Castleford, but we could probably survey the whole population of Askern in one afternoon.
Good point. I'd say there's double the present platform width available all together, but you'd still have to move track around and add pointwork, which pushes the cost upwards. Askern is just north of Doncaster; if you built a P&R station north of the old station, between Askern and Notton, you might have a chance assuming you could get passenger trains running regularly on that stretch. As far as I'm aware it didn't make the Reverse Beeching list.
In a way, the question really is "Why is Knottingley not as popular as Steeton & Silsden, or Horsforth, or Micklefield?" They all attract passengers from just over the border and have full (and sometimes extended) car parks. Without building some sort of Ferrybridge Parkway on the power station site(!) how can we make Knottingley more attractive to M62 travellers that would warrant greater investment in the station?