Hard to tell.Just an adjective, or futuros esse?
Presumably they mean that the sentence means that the mighty family of Aeneas and the glorious Pallanteum would exist in the future, rather than that they would be mighty and glorious in the future.And I'm not sure what Conington/Nettleship mean with "Futuros not to be taken with magnos and nobile. The two things which Carmentis predicted as in the future were the mighty family of Aeneas and the glorious Pallanteum."
But it looks like Servius and F&S are alone in that interpretation.ambitiosius quam si 'magistri' diceret: et rem personae in artem transtulit, ut si dicas 'opus est peritia medicinae', non 'medici'.