A large chunk of the Highlands is already a National Park:
https://en.wikipedia.org/wiki/Scottish_Highlands
Maybe Ballater - I wouldn't put that in the Highlands although Braemar clearly is!
Apart from along the Highland Boundary Fault I think the actual boundary of the Highlands is fairly fuzzy anyway :-)
http://www.bbc.co.uk/news/uk-scotland-highlands-islands-3380...