Obviously, Shattenkirk would have to come with an extension, but even then there would be challenges. For one, Krejci has a full NMC, so he'd have to agree to any trade, but I'm not sure he has any reason to. He's in a pretty great spot in Boston, as he'll be deployed in an offensive role with Bergeron and Backes taking all the hard minutes, whereas in St. Louis, he'd be counted on to play harder minutes.
Then there's the salary problem. The Blues are right up against the cap this season, and Boston would have to fit in an extended Shattenkirk along with extensions for Marchand, Spooner, and Pastrnak next year. You can help things a bit by sending Boston a player with another year on his contract (probably Sobotka) so that they have more flexibility near year, but they'd still be in trouble.
Assuming Boston has some plan to get under the cap for 2017-18, I could see something like Shattenkirk and Sobotka for Krejci, give or take some picks. But I still think the chances of Krejci waiving his NMC to go to St. Louis are negligible.