I think turning the Avista into the next generation ATS, in coupe and sedan form, is the best thing GM could do. It would put into production a concept that received an overwhelmingly positive public response and would give Cadillac a sorely needed car that turns heads and a new design language. As much as I loved the original CTS, none of the subsequent attempts of imposing the A&S design theme on other Caddies have been as successfully executed. The public hasn't responded well either. It's also dated now, IMO. Using the Avista and its design language for Cadillac's lineup would breathe new life into the brand.
Which brings us to Buick. Buick should be sharing cars and a design language with Opel. With things going well for Opel now there's no need for the brand to change design themes. That's not the case with Cadillac. The Avista wouldn't fit in with the rest of Buick's and Opel's lineup and I fear GM would let it wither on the vine if it weren't seen as a core Buick product. Buick also doesn't need a car like the Avista. Cadillac does and already has a similar car with the ATS. It just needs a makeover.