He's the current French heir and hates Richelieu's guts. Also, we just happen to have a bunch of Huguenot assassins running around. Perhaps they will make an attempt on Richelieu, and get King Louis instead. Richelieu, of course, will promptly get the hell out of Dodge. Defecting to the USE would be too obvious, but he could go elsewhere, possibly joining the Dutch Hapsburgs, or even try to carve out an empire of his own in the New World.
- With regard to this, a line at the end of 1635: The Papal Stakes indicates Gaston is also very, very anti-USE, as in siding with Borja against Pope Urban on the grounds that Urban's toleration toward the USE makes him unfit to be pope. If Monsieur comes to the throne, he may try another war against the USE ... possibly before France has modernized its forces sufficiently to win. Especially since Turenne and his bunch, as Richelieu appointees, are likely to still be on Gaston's fecal roster. He's implied to not be smart enough to forgive and forget simply because Turenne is the best military leader France has.