If runway 12/30 is permanently closed, a domestic and/or LoCo terminal could be built adjacent to the new Edinburgh Gateway Station and the tram depot. This would free up space in the existing terminal to allow remodeling.
There is also a considerable amount of space in the vicinity of the former RAF Turnhouse that could be developed. In addition, if the long-term carpark is relocated and the adjacent hangers are demolished, there's a lot of potential space for expansion in that area. Finally, there is plenty scope to extend the terminal towards the SE pier.
EDI could greatly improve and expand its facilities in various ways without needing to acquire land from the RHAS. The rapid growth in passenger numbers and routes is good news and will surely justify the continuing expansion of facilities.