This is a tough one, because you will generate a lot of largely idiosyncratic aspirations from us, most of which are probably not viable
Spot on; I confidently predict a lot of links between two random stations (with no evidence of a passenger flow between the two)...
However, I've a few wee suggestions:
1. Reading to Brighton. Either the old SWT way via Basingstoke or the FGW way via Reading. Doesn't even need to be frequent, but Reading is a big hub, and it would take a bit of traffic from Brighton that currently has to go through London to get anywhere
2. Edinburgh to Inverclyde/ Ayrshire. Would there be scope for a direct service a few times a day (e.g. link the DMU run Stranraer - Kilmarnock - Glasgow to the DMU run Glasgow - Shotts - Edinburgh?).
3. Edinburgh to the West Highlands (running a through service Waverley - Falkirk High - Queen Street - Oban?) - I appreciate that this is possible via the Sleeper, but there must be a daytime market for that.
Whilst the Welsh Assembly has meant services being diverted to Cardiff (e.g. the old Holyhead - Crewe and Liverpool - Cardiff are no more, but there are a lot of Holyhead - Cardiff trains), there's not been any trains linking Edinburgh (home of the Scottish Parliament) to previously unserved bits of Scotland (there is now one train a day on the recently opened Alloa branch, plus there will be direct trains to/from Airdrie etc come December)
4. Bradford to Morcambe. Morcambe used to be know as "Bradford On Sea". Whilst its not the attraction that it once was, I wonder whether diverting some of the Leeds - Morcambe services from/ to Bradford Foster Square instead would give a new market (okay, its possible to change at Shipley, but maybe not attractive for people with a lot of luggage)
5. Glasgow - Blackpool. TPE may be doing this once a week, but I think you could justify a much better service (especially, but not only, in the summer). Glaswegians are attracted to Blackpool like... (I'll let you fill in your own simile here)
6. Liverpool to North Wales. Similar to the Glasgow - Blackpool suggestion. North Wales seems full of Scousers, but there are no direct trains.
7. South Lanarkshire to Stirlingshire. There's a service from Motherwell to Cumbernauld, but the service from Cumbernauld to Falkirk Grahamston is the one from Queen Street. This seems strange to me as Glasgow to Falkirk is already well served by the Falkirk High service. Why not extend the Motherwell service to Falkirk instead, and provide a few links that avoid people having to go via Edinburgh/ Glasgow (e.g. this would allow people in Falkirk to change at Motherwell onto WCML services)? Future electrification would allow other routes like Glasgow - Hamilton - Motherwell - Coatbridge - Cumbernauld - Falkirk (which would give a few more "add on" links).
(that'll do for now - am trying to be realistic with routes where there seems to be some demand, rather than just give you a Fantasy List of places I'd like a service to from the town I live in now)