The highest priority one in my view should be extending from Headbolt to Wigan Wallgate, though I think I would only extend 2 of the 4tph. The layout of Headbolt (if a footbridge with lifts was added) does work with this as it's a single line with a freight siding on the Wigan side so you could easily have a bay for terminating services and a through platform. What you'd need to do is remove the current Wigan platform and concrete block and lower the track level down to meet the present ticket office side platform, and add appropriate signalling.
I really can't decide about Wrexham-Bidston. My inclination here is that being a much longer run which would make a different style of train with toilets more desirable would mean it would ideally make more sense for TfW to obtain suitable units to run a half hourly service terminating at the currently disused platform at James St, with a lift added to this platform. However it is a long battery run to Wrexham so this might not yet be feasible without extra electrification.
Liverpool Airport should be done by taking the WCML slow lines down what is now a road via a couple of Speke stations to the Airport. This is much easier to do because of the position of these lines with regard to ground level. 4tph of EMUs could then shuttle between Lime St and the Airport; if Wapping Tunnel ever gets done an Ormskirk-Airport service could replace this. This would provide a proper service at Mossley Hill and West Allerton too, with those stops taken out of the WMT services. It could even be considered to do it by way of tram-train which would be cheaper as you could cross roads on the level and run it down the central reservation of the existing roads rather than closing them to dig out a proper segregated heavy rail line. (Note tram, not fake tram

)
Helsby would be easy to do but only viable in conjunction with major housing development.
Ormskirk-Preston I struggle with, again it's too long and thin for 777s. I think extending to Burscough would be viable but it's really nothingness beyond that. Perhaps a sensible option would be 2tph to extend from Ormskirk to Burscough via the South Curve terminating at a new bay at Burscough Bridge, and a Southport-Preston service via the North Curve, taking up the local stops from the Manchester service.