Vast wild area (nearly all Highlands), linking existing

Vast swathes of Scotland are among the least populated in Europe [https://www.businessinsider.com/incredible-map-shows-scottish-population-2016-3?r=US&IR=T] and most areas are ecologically trashed wet deserts. What is there now cannot therefore be the defining factor - it must be about potential.
Britain has no true US- or African-style national parks - real wilderness. Some extirpated species need very large areas to be viable when reintroduced. So we need to dream on a huge, globally significant, 50 or more years timescale.
Reclassify almost the entire north, joining the Cairngorms contiguously to the entire west coast north of Oban, as the Scottish Highlands National Park in the current 'weak' mould (with cutouts for current and activity repopulating small centres) driven primarily by reforestation but permitting selective forestry, windmills, off-grid hutting and low impact activity and, within that, demark large sacrosanct core areas that exclude all development, motorised traffic and prioritise letting nature take its own course.

Historic response to climate and biodiversity crises on the scale that is needed, self-reliance (growth of indigenous timber industry & rural employment), rural repopulation, tourism, reconnection with the land, national pride, playing our global part.