It's pretty much guaranteed to be Winchburgh, being the only one that's actually funded. The next most likely, I'd say probably one of Newburgh, Abernethy or Bridge of Earn. Halbeath P&R station is also a possibility although the train would be slower and less frequent than the bus so I don't see the point (though they said that about Portway too). Bonnybridge could also be revived at some point.
I don't really think it's the climate for full new lines at the moment, the easier ones in Scotland have been mostly done so you're left with the fairly inconvenient to implement schemes such as Alloa-Dunfermline, Aberdeen-Peterhead/Fraserburgh or even Tweedbank-Hawick. Neither of them connecting more than 50,000 people to the railway, for probably at least a billion pounds each. A major housing development may put them higher up the priority list. Would be great but won't happen for a long time.
What about Grangemouth? It seems to be a very simple idea that could be implemented quickly
Theoretically the most likely new line, but a lot of logistics to sort out with that one, must be hard to implement given that it's a busy bit of railway. Best chance is an extension of the Glasgow-Falkirk (via Cumbernauld) service but without an Edinburgh link the business case wouldn't be too strong. Grangemouth is also a very low density town so any station location wouldn't be useful for at least half the population, since if you already have to drive to the station you may as well park at Polmont/Linlithgow.
