Originally posted by KnightofTempest
I would like to point out that Sigismund August did have an heir, as I've stated Louis II of Hungary and Bohemia was the heir. All that needs to happen is for him not to die at Mohacs.
No, I do not think so. Louis II died in 1526 at Mohacs. When that happened Sigismund August was only 6 years old. I doubt Louis was ever officially an heir of Polish throne.
I agree however, that had Louis II survived, he might have been a potential successor of Sigismund August. Question is would the Poles have wanted him? He had rather poor reputation, AFAIK. Besides, when Sigismund August died Louis II (had he survived) would have been 66 years old.
However, he could have had a son. Or better 2 sons.
Now imagine that worried about his sons' safety Louis II decides to send the elder to the Habsburgs (his wife's family), and younger to his royal cousin to Poland for education. Sigismund August, himself childless, takes a liking to the boy and treats him as his own son, eventually declaring him his successor. Louis accepts that, since he already has an heir. With some effort on the Sigismund August's part, Poles accept that too.
After Sigismund August death Władysław is crowned as King of Poland and Grand Duke of Lithuania, Władysław IV.
Now, what that means for the Polish-Lithuanian Commonwealth?
I think that streghtening of the Polish-Lithuanian union was more or less inevitable. Both countries lived in personal union for almost 200 years. Poles pushed for stronger union it since 1386 (union of Krewo), although they prefered simple incorporation of Lithuania into Poland. That was inacceptable to the Lithuanians. However, as I mentioned earlier, IOTL Lithuania had problems with Moscow. Unable to fight them on them own and with strong pressure form the king (and his authority as Grand Duke was stronger in Lithuania than king's authority in Poland) they had to agree to the permanent, more or less constitutional federation.
It is very hard to say how having an official heir would have influenced Sigismund August's policy. The process, IMO, would be somewhat slower, since the king didn't want the Polish-Lithuanian union to dissolve without common ruler after his death. With succession secured he might have acted more carefully and slower. OTOH it also very possible that he also would have wanted to leave his "son" better organized kingdom.
Personally, I believe that some kind of the union would be created in 1580s, similar to the OTL Polish-Lithuanian Commonwealth, although with some changes. There would be election of the kings as IOTL - but the kings would have been elected from among the Jagiellon dynasty (as it happened earlier); limited election pool would mean the every elected king would care only for PLC interest instead of their other kingdoms and wouldn't need to buy noblemen's votes with provileges and (often) empty promises. That already is a good thing.