Elizabeth Percy, Diana de Vere (daughter of the Earl of Oxford), Isabella Bennet (Css of Arlington), and those are just the heiresses that Charles II's bastards snatched up. Either way, a domestic marriage to an heiress means that a) James II/Charles III doesn't have to alienate lands from the crown for his brother; 2) the royal dukedoms that they were given return to the crown; and 3) most importantly, a wealthy wife means that Parliament doesn't have to pay out to support the prince - which means that the king doesn't have to ask parliament for money (for that, anyway).
A Brandenburger girl seems unlikely - Friedrich Wilhelm hated his stepmother (and by extension his half-sisters), he wouldn't want his stepmother to be getting all high and mighty because her daughter might end up as queen of England or at least (for a while) the second lady of the realm