Before Bernadotte was appointed crown prince, the Danish prince Karl August was actually Crown Prince for a while, but he suddenly died, leaving the succession open. Karl August had supported a Scandinavian alliance with three separate kingdoms under the same dynasty (i.e. not under the same monarch, but all being of the House Oldenburg). The French would never allow Bernadotte to keep the Swedish throne, since he was considered a traitor. The Danes on the other hand were allies, and would definitely get Norway back in a Napoleonic victory scenario. With Sweden lacking a monarch, binding them to the House of Oldenburg might not be far fetched at all.