Given the problems caused by Chathill (a lightly used local station to serve a small village on the busy/fast ECML) we shouldn't really be spend twenty million quid on building a station at Reston in the first place IMHO. See also Forum suggestions for stations at Beattock, Shap and other small places on fast main lines that have no local services suitable to stop there.
But, we are spaffing that money on Reston, so the next question is how do we actually serve it (given that there's no "local" service).
Ideally, there'd be some kind of service that was of use from getting from Alnmouth to Morpeth in one direction and Berwick in the other direction. It seems a shame that these towns have effectively no useful service (sure, there are some trains that stop at two of them but pretty irregularly) - but we are stuck in a position where there'd be outrage if Edinburgh lost (some of) its trains to London/ Leeds/ Manchester/ Birmingham - nobody wants to see long distance services slowed down - there's the First Open Access services on the horizon - slow freight paths exist (whether they are used or not) - so there's not much hope of finding space for a 100mph EMU with several intermediate stops.
And even if there were paths at the Edinburgh end (which is doubtful, given the North Berwick and Tweedbank services in addition to the English services), I don't think that Chathill would be getting any additional stops.
Maybe you could have two "fast" trains per hour from Newcastle to Edinburgh (bi-hourly stops at one of Morpeth/ Alnmouth/ Berwick/ Dunbar, so that they retain a fast service) plus two "semi-fast" per hour from Newcastle to Edinburgh (stopping at at least four out of Cramlington/ Morpeth/ Alnmouth/ Berwick/ Reston/ Dunbar/ one East Lothian station*, maybe in line with the peak flows - e.g. Cramlington getting a reduced northbound service in the morning so that more stops can be allocated to the Reston/ Dunbar commuters heading into Edinburgh, but Cramlington getting a half hourly service in the afternoon with Reston/ Dunbar dropping down to bi-hourly)...
...but that would affect the current number of fast services from Edinburgh to English destinations - as well as the complications of who'd operate such a service - especially given the "politics" - I can't see it being easy. So maybe Reston will just be served by extending some Edinburgh - Dunbar services to Berwick, with no change for the long suffering people of Northumbria.
(* - since this would be replacing the existing Dunbar - Edinburgh ScotRail services, I think you'd need to give Dunbar some stops at somewhere else in East Lothian)