I recently came across an anecdote in Peter H Wilson's book on the 30YW about Rudolf II. Apparently in 1591 he was conducting an experiment that literally blew up in his face, burning his cheek and singeing off the hair of his beard and eyebrows. Wilson dates this to September and says another observer standing nearby was killed. I found another source, a dispatch from the Venetian Ambassador to Prague, that dates the even to March 1591 but otherwise corroborates the story saying specifically that it involved heating large quantities of spirits and combining them with gunpowder. Easy to see how that could go wrong. So let's say the explosion is a little more violent and Rudolf is killed instantly or mortally wounded. Aside from making his death a great piece of trivia for future generations it would seem to have pretty serious consequences for Austria and Europe in general.
In 1591 his next eldest brother Ernst is still alive, and still governing Austria so I assume he becomes Emperor but then what? Ernst died in 1595 OTL so he may not have many years left. Does he marry Isabella Clara Eugenia or now that he's Emperor does he marry a German princess (maybe Maria Anna of Bavaria, OTL wife of Ferdinand II), or even an Austrian cousin (Anne of Austria, otl wife of Sigismund III)? Ultimately the brothers conflict between Matthias, Maximilian, Albert and their cousins in Styria may still be a problem if Ernst dies a few years later without a clear heir. On the other hand if he dies with a minor son only a couple years old that's not much better.
Looking elsewhere does Philip install Albert as regent in Brussels in 1594 instead of Ernst, and does he marry Isabella to him earlier (assuming she doesn't marry Ernst). In 1591 I think she's still a major factor in the French succession struggle. Also there's the impending war with the Turks, OTL's Long Turkish War. Is there any chance Austria could do better and recover more of Hungary? Or if Ernst died in the middle of the war and threw the Austrian side into chaos would they do worse, maybe another siege of Vienna?
In 1591 his next eldest brother Ernst is still alive, and still governing Austria so I assume he becomes Emperor but then what? Ernst died in 1595 OTL so he may not have many years left. Does he marry Isabella Clara Eugenia or now that he's Emperor does he marry a German princess (maybe Maria Anna of Bavaria, OTL wife of Ferdinand II), or even an Austrian cousin (Anne of Austria, otl wife of Sigismund III)? Ultimately the brothers conflict between Matthias, Maximilian, Albert and their cousins in Styria may still be a problem if Ernst dies a few years later without a clear heir. On the other hand if he dies with a minor son only a couple years old that's not much better.
Looking elsewhere does Philip install Albert as regent in Brussels in 1594 instead of Ernst, and does he marry Isabella to him earlier (assuming she doesn't marry Ernst). In 1591 I think she's still a major factor in the French succession struggle. Also there's the impending war with the Turks, OTL's Long Turkish War. Is there any chance Austria could do better and recover more of Hungary? Or if Ernst died in the middle of the war and threw the Austrian side into chaos would they do worse, maybe another siege of Vienna?