I'd go for a 16 top league, 3 x 12 lower leagues and, as feeders, a Highland league, an East central and a West central
Bottom two in top league play off with top two in next and so on to bottom league where bottom 3 play off with winners of each of feeder leagues. Relegated teams from bottom league slot into feeder leagues depending on geographical location.
Where would the additional teams come from? There are plenty junior and amatuer teams in the country who could hold their own in the lower divisions of the scottish leagues.
As things are at the moment all lower scottish league teams can enter 3 cup competitions. Scottish Cup, Challenge cup and CIS cup. Premier teams can enter Scottish and CIS so why not, as well as the league competition introduce a Premier league cup k.o competition. This would provide some additional games to offset the reduction to 30