The Birkenhead North plan needed additional rolling stock.
Cutting back to Bidston made the service work (just) with two units (at 1tph).
If 230s can do the route faster, there might be enough turn-round time to extend to Birkenhead North again - but needs to be proved.
Originally, the LNER/BR service went to Seacombe, but diverted to New Brighton when the Seacombe branch closed (for the Wallasey tunnel access road to be built).
However, the future plan is 2tph, one all stops and one skip-stopping. This will still require four units but will allow a longer recovery time at the termini. Basically, two units will do all stops one way and skip-stop in the opposite direction, whilst the other two will do the inverse. This could allow re-extension to Birkenhead North, although it would require the skip-stop service to skip more stations than terminating at Bidston would. The running on a busy stretch of Merseyrail could cause issues however. Ideally, it'd be good at have the Borderlands services on their own track between Bidston Junction and Birkenhead North, with their own terminal platform. Perhaps a single track south of the existing tracks and then skew the Merseyrail services to serve platforms 1 & 2 with the Borderlands terminators on 3. Birkenhead North as an interchange offers double the Liverpool-bound frequency than Bidston does plus the addition of the New Brighton services. Add to this the doubling of the frequency and the line would become a lot more useful.
Running non-standard diesel stock into the Merseyrail underground system is not going to happen. Merseyrail is a metropolitan system and should have a standard (or very similar) fleet, all electric and under one operating company (ideally, there'd be vertical integration too). The only way I'd even entertain the idea of incorporating the Borderlands lines would be if the line was electrified (partially or wholly) and the services were transferred to Merseyrail. But even then, I'm not certain the line is necessarily a good fit. For starters, the length of the line would make it by far the largest branch on the system (and no, Southport-Hunts Cross is not a single branch despite through services). It's a predominately rural line serving mostly very small places with the only place of any notable size being Wrexham at the far end. The Halton Curve services will allow direct services into Lime Street with at least a similar and probably notably quicker, journey time.
I personally think Chester & Southport are about the absolute limit of the sort of system Merseyrail is, considering its metro-type rolling stock with no toilets and the call at every station convention. Sure, you could argue to change the system by adding fast trains, toilets (and no doubt first class and a buffet trolley also) having trains with a more long/medium distance layout which prioritise top speed over acceleration but then you compromise the inner network who will surely have lower frequencies due to the fast services and trains which aren't as appropriate for metro services or a mixed fleet of metro trains and longer distance which would be an inconsistent experience for an inner metro passenger and more of a logistical challenge than operating a uniform fleet. You could split the line as stations at the south most likely mainly use the line to commute to Wrexham anyway rather than head northwards and a shorter line may be a better fit for Merseyrail and allow a metro-type service for the Wirral stations.
The main question then is where you break the line - with Shotton, Neston, Heswall and a new station at Woodchurch all being common suggestions. Shotton is probably similar, distance-wise to Chester so could be just about doable, plus it provides interchange with the North Wales Coast line. Neston is more urban and in a local authority area that already has Merseyrail (Cheshire West) and therefore an extant agreement on Merseytravel tickets. Heswall is actually in the Merseytravel area whilst Woodchurch has been Merseytravel's most recent preference due to its road links. The problem is, the further south you cut the line, the less viable the resultant diesel island would be and the further north you cut it, the less it's worth bother making the alteration. Merseytravel's Woodchurch propsal would mean a relatively short branch of the Wirral line off Bidston with only two intermediate stations at Beechwood/Noctorum (new station) & Upton. If this could justify 4tph, then it would maybe be worth it but I've often seen it envisioned with only 2tph which doesn't seem worth it to me and would mean a worse interchange point from the remaining diesel line than Bidston currently is and certainly what Birkenhead North would be.
There's probably no ideal solution but I'm minded to support leaving the line as it is; ideally with an extension to terminate at Birkenhead North. A 2tph frequency with a change at Birkenhead North which offers 8tph into Liverpool, as well as 4tph to New Brighton & 4tph to West Kirby would be much better than 1tph with a change at Bidston which offers 4tph to Liverpool, 4tph to West Kirby and 0tph to New Brighton.
