Journey to the East
Kill Russia during the Troubles, letting the PLC annex large swathes of Russian land. This land will naturally be absorbed into the Lithuanian parts of the Confederacy and will also preserve their power base.
However the systemic problems of the PLC's government aren't going to go away without a strong need for reform, significantly weakening the PLC's ability to project power or even just defend itself. Prussia+Austria, fearing this larger neighbor's power should it ever manage to reform it's government, start picking away at its western border. In Particular, Prussia will be moving right into and attempting to take the core of Poland, while Austria will naturally want Krakow and Galicia.
With the Polish part of the PLC vastly reduced in both size and wealth, the Lithuanian part will become far more important. If you get the Polish-Prussian Border close enough to Warsaw, then the capital will probably be relocated to somewhere in Lithuania, further solidifying Lithuania's control of the PLC Government.