Despite the prospect for life extension programmes to current airframes (including Saab) there may well be potential for an innovative manufacturer to offer a cost effective current technology aircraft to the existing market segment not serviced by ATR or Bombardier.
Sorry, but the window of economic viability on any sort of scale to support a 50 seater has passed, particularly one with 6,000 ESHP (thermodynamic) engines, has passed.
70 to 90 seats is the current point of entry for a turbo prop, 100-130 seats for a jet..