Absolutely easiest way to fullfill this challenge is to make PLC ruled by Piasts. Sigismund the Old planned to marry his oldest daughter Hedwig to last Dukes od Mazovia: Stanisław and after his death, his brother Janusz III. They both died young, but if any of them could live a few years longer (with their lifestyle long life is unlikely), Say Janusz lives 5 years longer, marries 17 years old Hedwig in 1530, sees birth of their son Konrad IV in 1531 just before his death (better if their child is not raised by Janusz). If Sigismund Augustus still dies childless, then his Mazovian nephew would be his obvious successor. That means 1573 election is pure formality and there is no such long interregnum. Also with king, who is not foreigner, Jan Zamoyski would not became de facto co-ruler of PLC and would not have chance to shape PLC as much as IOTL (for example by making office of hetman given for life, which was disastrous for PLC's army and changed hetman into more political than military figure). Zamoyski was really destructive person for Commonwealth, easpecially during early Sigismund III's reign, when he was conflicted with King. Also, Mazovian King of PLC, unlike OTL monarchs, would have large hereditary duchy within PLC, which would strenghten his position (Duchy of Mazovia is not Jagiellon's GDL, but is still better than nothing). Also without incorporation of Mazovia Warsaw would not be capital (propably Sejm would be held in Lublin) and numerous Mazovian poor nobility would not get as much influence as IOTL, without Mazovia, Crown would have much less % of nobles-average % of nobles for Poland without Mazovia was 3-6%, addition of Mazovia with 20% of nobles caused nationwide rise to 10%)