The Sutherland Trail may be of interest to you. Another option which is actually the north-west coast and doesn't go anywhere near Caithness is the northern end of Cape Wrath Trail. There is no set route but if you Google it you'll see the options.
I would suspect walking along the north coast would be very time consuming as there are numerous inlets, even the road journey takes a long time.
On Friday 26 April 2013 I was off to Teanga Brideig, Glen More, Isle of Mull to climb the twin Grahams, Cruachan Dearg and Corra-bheinn along with the Graham Cruach Choireadail.
Despite some wintry showers I managed to take several photos which can be viewed from this slide show.