I'd vote for anyone who made services in north west England more reliable by splitting the routes around Piccadilly so that all Airport services were all Castlefield services and vice versa - i.e. make the Airport - Castlefield operationally independent of the "main shed" services (other than freight, which we can't really do much with) - that would be huge improvement (Sheffield would lose its through trains to Liverpool/ Manchester Airport but that's a price I'm happy to pay for more reliable services)
One extension that I was slightly surprised not to see was a thorough service from Yorkshire to Skegness - EMT were good at allocating stock to meet demand (HSTs to Lincoln market in December, seven coach 222s to Liverpool for the horse racing, HSTs from Nottingham to Skeggy in the summer) but they never used the Neville Hill HSTs for Skegness services
One other Lincolnshire link would be some kind of service from the Skegness branch to either the ECML at Peterborough or to Lincoln (I'm generally not fussed about "county towns" but there's no denying that Lincoln is the big draw in the area)
extending the North Downs service to Brighton. This would not only provide a better service from Redhill to Brighton, but also to large centres such as Guildford and Reading. Something that obviously can't happen due to capacity constraints south of Gatwick.
That sounds s good idea on paper - I don't subscribe to the "Brighton must be back on the XC map with through services to Liverpool" debates on here but Brighton did have direct Reading services via Kensington Olympia (XC) and Botley (SWT) - Reading is a significant place in its own right as well as being a hub for a wide range of services (towards Bristol, Birmingham etc) - so some way of providing a through train from Brighton to Reading sounds like something desirable in the long term.
Extending the Doncaster-Scunthorpe stopper to run through to Cleethorpes in order to give Grimsby and Cleethorpes a second train an hour to the ECML.
YES!
Simple, practical, beneficial, fairly cheap too.
The Grimsby/Cleethorpes threads on here are full of grand ambitions for regular London services, a significant increase in the number of trains via Brigg etc, but people don't seem interested in something as useful as doubling the frequency from Grimsby to Scunthorpe and the ECML at Donny.
Now that the Doncaster - Scunthorpe service has been separated from the old Scunthorpe - Sheffield - Lincoln - Sheffield - Adwick - Sheffield - Scunthorpe diagrams, it should be easier to path a Doncaster - Cleethorpes service co-ordinated with the Manchester Airport service to give a reasonably spaced two trains per hour through Scunthorpe.
Manchester Airport is very much an International Airport
If, by that, you mean "it is an airport with international services" then the same could be said of many airports, but we don't cripple services in the region by trying to provide direct services to everywhere - e.g.there's no suggestion that Cardiff Airport has to have direct trains from all over Wales... there's no suggestion that Birmingham Airport has to have direct trains from all over the Midlands (it happens to be on the Coventry corridor, so gets the trains that would stop there anyway)... Merseyrail don't feel the need to extend some services from Kirkby/ Ormskirk/ Birkenhead to John Lennon Airport... Newcastle and Edinburgh airports have international flights but rely on the Metro/Tram instead of trying to provide through trains from everywhere in a hundred mile radius...
Similarly, the Manchester Metrolink and Tyne'n'Wear Metro stick to just
one service to the Airport branch, rather then trying to provide through links from every other branch on the network (anyone from Bury/ Rochdale/ Ashton etc can change in central Manchester)
Sometimes its easy to serve an Airport since it lies on a busy line (e.g. Luton, Gatwick, Birmingham, Southampton), but the complications of Manchester mean it's on a branch that makes it more complicated to serve (effectively meaning one extra unit required to provide an hourly extension).
Tyne Valley services to Chester-le-Street and Durham.
Obviously, there's a host of practical reasons why this wouldn't actually happen.
Some kind of through service from Darlington/ Durham to the Metro Centre sounds a good idea (if paths, stock etc)
The Scotch and North Welsh services to Manchester should terminate at Victoria, with 1 or 2 sidings built just east of Victoria so that they don't block the through platforms during the layover period.
No need for new sidings, just dump the train five minutes away in Newton Heath - staff can have their PNB there