Building on earlier ideas, how about if Saxony - dynastically linked with the Grand Duchy of Warsaw - didn't stay loyal to Napoleon during 1813 as in OTL, but instead turned against him and negotiated with the Coalition to keep their gains (as Bavaria and others did)? If combined with a weaker Prussia for whatever reason, Saxony could plausibly play a major role in the unification of Germany; after that, the Wettins would rule as dual monarchs over Saxony (or the whole of Germany) and Warsaw/Poland. Indeed, supporting Polish nationalist ambitions could potentially be a very useful tool for Germany to try and undermine Russia and/or Austria, depending where the borders ended up at the Congress of Vienna.