So I was at Waverley today and thought I'd have a look about.
Platform 13: Definitely can't be extended even for 6 car trains without demolishing the south ramp.
Platform 14: Might need the final 30m or so before the current buffer stops slewed slightly to the south but otherwise looks possible to extend to 8 car length without changes to the ramp. Will need demolition of nearly all the buildings around platforms 11 and 12 including the sleeper lounge.
Platforms 5/6: extending the old motorail platforms to cater for full length 91s / IEPs looks very possible, there is plenty of space for 2 tracks where the old taxi rank was and this is now a very quiet, underused area of the station. Probably makes sense to run right up to the east side of the main pedestrian walkway.
Linking platform 6 to platform 12: This would be very tricky to fit in with the layout around the new escalators from the pedestrian bridge and would severely affect pedestrian flow from the main concourse to platforms 11, 12 and 5. Probably not viable or necessary.
One tricky issue to resolve would be revenue gating an extended platform 12 as it would have to be accessed from the platform 11 area where there would be a mix of long distance passengers.
If you're moving more long distance trains into 5/6 and continuing the increase in footfall to the east end of the station then options need to be explored to open up access to the main booking hall from the east and south sides of the station. It may also be worth extending the Calton Road walkway southwards over platforms 2-9 and possibly through to New St car park / Waverley Court area.
Once you've done these improvements if you wanted more platforms, 400m platforms for HSR etc you're going to have to move the entire concourse up to the pedestrian bridge level, demolish both ramps and run long through platforms in place of the current booking hall. With the grade 1 listing this would be a very hard sell unless facilitating something very major like European Gauge HS trains etc.