I've been toying with this for a while, and now I'm throwing it, partly inspired by Anaxagoras "God is Frenchman".
At the beginning of the War of Austrian Succession, Austria was about collapse. Maria Theresa later managed to recover, mostly because of the looseness of the coalition of her enemies - especially the jealousy between Bavaria, Saxony and Brandenburg.
The Bavarians and their French allies failed to take Vienna when they could, eager to secure Prague and Bohemia against Saxon claims instead. Frederick had advised against such a course of action.
Let's say that French diplomacy plays its cards better, talking the three German states into a working alliance and a viable partition plan. Prussia is given all of Silesia, Saxony gets Bohemia and Moravia, Bavaria annexes everything else the Hapsburgs had in Germany, Spain all the Austrian holdings in Italy. France's share might be the Austrian Netherlands.
The Franco-Bavarians march on Vienna steadily leaving the Saxons to try their chances in Bohemia.
The Hungarian diet sees the writing on the wall and fails to support Maria Theresa with the necessary manpower and money.
Britain and the United Provinces are dragged into the war big time, and earlier. Prussia sees the advantages of the situation and Hannover is overrun.
France and Spain crush Savoy and sweep through Italy essentially unopposed, carving the Duchies of Milan (with Parma and Mantua) and Tuscany for Philip V's sons and throwing some pieces to Modena, which is their closest ally in the peninsula.
Russia is busy with a war with France-supported Sweden and her own succession issues, Poland is paralyzed by the usual conflict between the Crown and the Sejm, aggravated by Stanislaus Augustus controlling Bohemia as well.
Britain tries to work out a diplomatic solution to get allies on the continent, and manages to keep Saxony-Bohemia-Poland relatively quiet and to make Hungary figh on until a much mor powerful French army invades Hungary proper, say in 1743.
Around 1745 Maria Theresa accepts a peace that leaves her and her husband as ruler of Hungary alone, without a strip of land roughly corresponding to OTLs Burgenland that the Bavarians manage to add to their Austrian conquests. Piedmont surrenders about the same time, giving Sardinia to Naples, Nice and Savoy to France, and some minor areas to Genua, another French ally.
Britain and the Netherlands fight on, but in 1747 the Netherlands are overrun completely as the French army enters the country in support of the Party of the Prince (how plausible this bit would be?).
With Spain freer to focus on the colonies for the las part of the war, Britain has even worse time at sea and in the colonies than OTL. Louisbourg is not taken, but Madras is still lost and maybe Georgia as well.
In 1748 a seriously weakened Britain accepts a humiliating peace.
My original idea was a treaty where all the Austrian Netherlands, the Dutch Flanders and maybe even Brabant and Limburg are annexed by France, while Hannover is incorporated into Prussia, who accepts to give France the bits of the Prussian Guelders west of the Rhine too. In the colonies, most of the Sugar Islands are lost, though probably Britain keeps at least Jamaica and Barbados; same thing for British settlements in Belize and Miskitia. Georgia is ceded to Spain and Acadia given back to France, who also keeps Madras.
But I guess that such terms are too harsh for any British government to accept, and they'll might try another round, maybe doing some mess in the Netherlands. The idea is that at this point it would be a failure, and France gets what she wanted.
Britain would surely seek a rematch in a few years, probably allying with Saxony, Hungary and maybe Russia vs. a compact of France, Spain, Bavaria and Prussia.
Thoughts?