How would you serve the 3 stations at the Manchester end that currently have a limited service- Trafford Park, Humphrey Park and Chassen Road? Leave them with 1tp2h or enhance to hourly or better?
By making one of the current semi-fast trains slower (e.g. the Airport-Liverpool, calling at something like Flixton, Urmston, Irlam, Birchwood, Warrington Central, Warrington West, Hunts Cross and South Parkway), you give the extra stations faster links, but also create space for one of the existing stoppers to also be slower (as it now won't be caught up as quickly) so could possibly pick up these stations hourly.