The question really boils down to seat-km cost. Other than in a few special cases, the days when yields on regional routes were high enough to cover the costs of a 34 - 50 seater - whether jet or turboprop - are gone. In the US cost-plus contract flying for the majors has either gone or become hugely unprofitable, and in Europe and other places the advent of pile 'em high sell 'em cheap LCCs has severely limited the opportunities for traditional regional aircraft flying. It seems that 70 seats is around the minimum that can deliver competitive seat-km costs and, although the ATR42 remains in production, it is being comprehensively outsold by the ATR72.
Therefore, it is is difficult to see where a warmed-over SB20 would gain traction - the nature of the beast means it needs to use its speed to gain additional rotations per day to compensate for its higher costs - just like the Q400 vs the ATR72 - and also operate in an environment where it can attract premium yields. There just don't seem to be that many such combinations around. All of which would seem to indicate that, if they are to re-enter the market, Saab would look at an all-new larger aircraft. But, if they did, ATR and Bombardier would be there first with much lower cost derivatives no doubt using the same engine technology.
Interesting times!