Buick is already expected to bring over the Opel Insignia wagon as a Regal wagon. Its success could result in further wagon options from Opel bring brought over under the Buick brand.
When Buick gets their version of the new Astra, it will debut in US-friendly sedan form with plenty of Buick differentiation (think waterfall grills, etc). The wagon could help expand this low-end of the Buick family (along with the upcoming coupe) to boost Buick's profile with a much younger buyer than has been the norm for many years.
In Europe, the Astra Sport Tourer is available with a wide range of diesel and gas engines from 95 to 180hp, expect a US version to play mainly in the higher end of that HP range and exclusively with gas engines. Fortunately, given the availability of the Insignia-based Regal in manual transmission form - there should be a manual option in the Astra-based range as well.
Expect the wagon to be a late introduction (several months after the sedan version, if at all) likely at the same time as the coupe (again, if it is made available to US buyers).