Improvements that could be possible with a larger station, that have been mentioned in the press, are doubling the frequency on the Coleraine line, with one train per hour from Belfast going on to Derry (as at present) and the second running through to Portrush. As well as doubling the frequency as far as Coleraine it will also remove the need to change at Coleraine for Portrush. (There are a lot of passengers for the Portrush line so through trains would be very popular).
Translink have said in the past they are open to running a commuter service from Belfast to Lisburn and up the currently rarely used line to Antrim. But they require Government finance both to run it and to renovate the mothballed stations.
Likewise there’s a fairly good case for re-opening Portadown to Armagh. But again it needs to be funded.
They do appear to have funding for replacing the Enterprise sets and running an hourly Belfast - Dublin service from around 2029. Some of that funding is from the EU.