There will be no significant new 3rd rail extensions. No one's suggesting the Midland Main Line be electrified with 750v DC. It also depends on what you consider 'short' extensions. Like all directives, it's open to interpretation. That's quite deliberate and one is supposed to apply common sense when referencing it. I consider the few muted Meseyrail extensions as 'short' for example and so see no need why they can't be 3rd rail, bar extension to Warrington. Some have read the same thing as 'no new third rail whatsoever' though and not only that but also any already extant should be changed over.
Consider this: Despite being mooted as an easy and simple extension of some 1.5 miles to instate the Headbolt Lane station, it was not done. Could the larger part of this reason be because it was to be 3rd rail electrified to fit in with the existing infrastructure and rolling stock. Furthermore, given a future scheme to Skelmersdale has been in the pipeline for some time, could the idea that a piecemeal creeping 3rd rail extension was a likely result of any agreement to build Headbolt Lane not have been additional reason for it not going forward. More recently the planned electrification of Bolton to Wigan has in my opinion only furthered the case that any 3rd rail extension beyond Kirkby (the most likely candidate) will not be accepted and thus how 'short' a section must be to be considered 'short'.
As others have stated I don't see any further 3rd rail electrification unless both ends of said extension are already 3rd rail. With the exception of tunnels where the costs of re-boring would make the cost prohibitively expensive and even then I think it more likely to not happen than be 3rd rail.
Therefore, it is my strongly held opinion that any Kirkby - Skelmersdale / Wigan extension of Merseyrail will be via OHLE at a time when an infill Salford - Southport is done. Given at this time the track between Fazakerley and Rainford will undoubtedly be redoubled, it will also be re-instated with OHLE and thus the changeover point has moved inwards at least as far as Fazakerley if not Rice Lane.
As has been mentioned above by others, potential Chester & Southport OHLE, the issue at Hunts Cross, the complications of 3rd rail beyond Ellesmere Port, the desire to bring the Borderlands line, Burscough South Curve, and extensions to Skelmersdale into the network show that almost all sections of the Merseyrail network are vulnerable to the gradual replacement of 3rd rail.
The higher ongoing cost of maintenance and losses combined with the potential to distribute power over a significantly wider area from centralized locations, in turn requiring further reduced maintenance and manning. Will always be enough to bias the balance in favour of OHLE over 3rd rail for as large a portion of the network as possible.