Here's what I think.
It's obvious that, for now, a 12-team SPL is good enough for the country. Actually, our league structure is good enough as well. What we need is more funding for the other teams [i.e; Dundee United, Aberdeen, Inverness CT, etc.] or to impose a nationwide transfer ban for at least a season to see if the other teams can improve and maybe even bring through some good young talent.
As for bringing in junior teams? No way. Junior teams play some Third Division teams in the Scottish Cup and get beaten nearly all the time [main exception was when Spartans got to the Quarters.] If junior teams WERE to be introduced, then I think it should be on relegation/promotion rules, kind of like the way it goes from First to Third divisions.