Not to distract from the main idea but wouldn't a son by Maria Josepha of Bavaria be more useful to Joseph II's ambitions? At least with regards to expansion within the HRE.
As to whom the boy would wed, I think that the typical candidates i.e. Welf, Bourbon (France, Spain etc), Wettin & Wittlesbach are more or less dead weight at this point. None had proven beneficial in the long run.
A Russian princess (Grandduchess Alexandra Pavlovna, daughter of Paul I) maybe best as Russia was the best ally Austria could hope for vis-a-vis Prussia at the time. Though considering the religious connotations (she refused in otl to convert to Catholicism when she married Leopold II's son, being Holy Roman Empress may change her mind though) unlikely to take place.
Internally, I honestly can't think of a German princess whose connections at this point would help further Austrian ambitions. The House of Welf was firmly in the Hohenzollern camp. Saxony though influential, with existing marriages still sided agx Austria/Joseph II, Bourbon's are likely in the throes of the Revolution or if they avoid it too mired in debt & Austrophobic (a least the French public & court) to be of any use.