As already said, spoilt for choice, away from the Central Belt almost anywhere in Scotland has great touring potential. Just look at a map and pick out a route missing out the trunk roads. Make it up as you go along. You can always use trains and I guess with a folding bike some coach services for some legs. What are your other interests ? e.g. If archeology is your thing then Orkney and Shetland are a must. If castles and gardens then Royal Deeside and Aberdeenshire take some beating. If amazing sunsets and spectacular sea views then the west coast will be just the ticket.
Or perhaps base on particular places you want to visit e.g. the Callanish Stones on Lewis, or the Cape Wrath lighthouse, or Skara Brae on Orkney ?
Or perhaps base on a route that someone else as followed, e.g. places of note in the Bonnie Prince Charlie/Jacobite story, or places mentioned in favourite books, or places featured in favourite films/tv. (For me I had to visit the telephone box used in Local Hero).
Or base on food and drink ? E.g. Speyside whisky, Cullen Skink, Arbroath Smokie, Aberdeen Angus, West Coast langoustine etc