That's the rub. Nobody buys coupes unless they're buying niche cars. Buick doesn't want a niche car. They want a big seller that they can put a halo on with the GN and GNX trim packages.
The path that makes most sense is what Dennis mentioned: a new Regal based on the Holden Commodore/Chevy SS platform. It will be four-door, because that's what sells. Americans are fat, and fat people don't like crawling in and out of the back seat of a coupe when they go to lunch with their friends. People don't want to struggle to get kids in and out of a child seat. I often take my daughter to car shows, and wrestling her seat into the back of my GN, and then getting her in is a PITA. These are all things that factor heavily in a family's choice to buy a car, and Buick wants them to buy their cars. They want to sell hundreds of thousands of $35K base four-door Regals to people pulled into the showroom by a $45K Grand National and a $55K GNX.
You guys demanding a coupe need to chill out. Pontiac tried it with the GTO. It failed. They won't do it again. If you want a coupe, get a Camaro or a Corvette.