Great idea, but the returns would be hard to justify the investment required to even get it to demonstration stage, let along certification. Saab nowadays is a military contractor so supporting the legacy commercial turboprops will likely go only as far as getting them to the end of life. Given the limited customer base, which is pretty much just Rex, unless Saab (or someone else) wants to proactively asset manage a '340NG' and create the market for them, it would be hard to justify in Stockholm.
Re-certification would only apply to a major change like changing the power plant, and the associated major work. The original SAAB 340B can be CAT II certified, it's already fitted to many of them, refitting the cockpit to glass is already an option. Renovating the airframe and fitting it out with modern interiors, ANC etc are all just engineering orders, not re-certification. Some of the US 340s were retrofitted with freon air conditioners in lieu of an APU as an example. And there are already approved low weight interior options, seats and fittings available for fitting.
An interesting idea. So for 60 airframes that would be between $300 and $600million. Add in the purchase price of the airline, that is a large investment in a loss making business.
It probably needed to be done 10 years ago, however, what options are there? A used ATR 42 will cost $10-$20 mil, a new one $20-$30mil, and they have a woeful reputation for extended life operation. I assume the new Dornier will come to market in the $20-$30 mil area as would a new SAAB 340NG. Those numbers are just the cost of doing business in aviation, you borrow the money and hope you can pay some of it down in the 20 years of operating, basically what every other airline does, except with much bigger numbers.