We can slightly modify the question of Ladislaus the Posthumous von Habsburg to more historical. Because we should create something like "King Burgundy dies" for Middle Europe but more twisted
For first Bohemia should start with Ladislaus the Posthumous von Habsburg as King at the age of 4 years with regent Jiri z Podiebrad at the age of 24 years.
Hungary should start with Ladislaus the Posthumous von Habsburg as King at the age of 4 years with regent Janos Hunyady at the age of 57 years.
In year 1453-1454 (before king will be have 15 years) Bohemia should have event with choose between "stay on the throne Ladislaus the Posthumous von Habsburg" (little chance for AI and mayby with effect of Hussite Rebelion or/and growth unrest in whole contry) or "allow the regent to take over the crown" (with effect Jiri z Podiebrad is new king, legitimacy is set to 0 and -200 points of relation with Austria)
In year 1453-1454 (before king will be have 15 years) Hungary should have event with choose between "stay on the throne Ladislaus the Posthumous von Habsburg" (little chance for AI and mayby with effect of growth unrest in whole contry) or "Matthias Corvinus Hunyady will be our King" (with effect Matthias Corvinus Hunyady is new heir with dathe of birth 1443, legitimacy is set to 0 and -200 points of relation with Austria)
If Hungary would choose the King from own dynasty, will should get special country modifier (+10-20% Morale of Armies) and -10% Regiment Costs and maybe something else until death Matthias Corvinus. We know that historically Hungary ruled by Matthias Corvinus was very strong and had "Black Army of Hungary".
If Bohemia would choose Jiri z Podiebrad as new king, will should give event for Austria with choose between "This is an insult" with "Restauration of Union" CB for Bohemian throne and permanent claims for all Bohemian provinces (why in next part) or "Unfortunately, we have to accept that" (little chance for AI) with Prestige -50.
If Austria would choose "This is an insult", will should get next event with options for all current rivals except Hungary [if is set as rival] ("Remove rival XXX and set Bohemia as rival")
If Hungary would choose Matthias Corvinus as new king, will should give event for Austria with choose between "This is an insult" with "Restauration of Union" CB for Hungarian throne and permanent claims for all Hungarian provinces (why in next part) or "Unfortunately, we have to accept that" with Prestige -50.
If Austria would choose "This is an insult", will should get next event with options for all current rivals except Bohemia [if is set as rival ] ("Remove rival XXX and set Hungary as rival")
It will cause that Hungary could be declare war against Bohemia but without Austrian help for Bohemia (as it was historical).
Of course if they all (Bohemia and Hungary) were keep Ladislaus the Posthumous von Habsburg as king it shoud cause Bohemian-Hungarian Union under Ladislaus the Posthumous von Habsburg without rivalry with Austrian but rather with friendly.
Now let's go to the Polish side of this case, for first Poland should start with permanent claims for all Silesia provinces (it will be caused Polish-Bohemian rivalry attitude and is historical, because Silesia was part of Poland in the Middle Ages).
Next, if:
- Polish-Lithuanian Union exists or Poland is Great Power
- Polish dynasty is Jagiellon
- Bohemia was denied the Habsburg's monarch
- Austria has set Bohemia as rival
- Is year between 1470-1500
Bohemia should have possible national decision, "offer a crown to son of "XXX Jagielon" from Poland and ask Jagiellon dynasty of protection against Habsburg dynasty".
This decion should cause event in Poland with options:
1) "take the Bohemian Crown" with effect XXX Jagiellon is ruler of Bohemia, prestige +X, remove Bohemia as rival (if is set) and Bohemia remove Poland as rival (if is set) +100 relations with Bohemia, Poland and their subjects remove claims for all Bohemian and their subjects provinces, Bohemia and their subjects remove claims for all Poland and their subjects provinces, Poland return all Bohemian or their subjects cores (if have) and Bohemia return all Poland and their subjects cores (if have), Poland gets -200 relations with Austria.
2) "Habsburgs are very powerful and they are our friends" with Poland-Austrian relations +X and -X for Poland-Bohemian relations and -X prestige for Poland.
If Poland would choose first option, Austria will should get an event with options for all current rivals "This is an in insult, remove rival XXX and set Poland as new rival" with reltaions -200 with Poland and prestige +X or "Unfortunately, we have to accept that" with prestige -X.
Next, if:
- Polish-Lithuanian Union exists or Poland is Great Power
- Bohemian dynasty is Jagiellon
- Polish dynasty is Jagiellon
- Hungary was denied the Habsburg's monarch
- Austria has set Hungary as rival
- Is year between 1490-1500
Hungary should have possible national decision, "create personal union under XXX Jagiellon from Bohemia and ask Jagiellon dynasty of protection against Habsburg dynasty".
This decion should cause event in Bohemia with options
1) "take the Hungarian Crown" with effect makes Hungary lesser partner in personal union, prestige +X, +100 relations with Hungary, Hungary and their subjects remove claims for all Bohemian and their subjects provinces, Bohemia and their subjects remove claims for all Hungary and their subjects provinces, Bohemia return all Hungary or their subjects cores (if have) and Hungary return all Bohemian and their subjects cores (if have), Bohemia gets -200 relations with Austria.
2) "give Hungarian Crown for our younger brother" with effect XXX Jagiellon is ruler of Hungary, Hungary and their subjects remove all claims and return cores for Bohemia, Poland and their subjects and vice versa, Hungary gets +100 relations with Bohemia and Poland and remove their as rival (if are set).
Last step of this modification is national decision possible for Poland "Jagiellons-Habsburgs agreements in Vienna" with requirements:
- Polish dynasty is Jagiellon
- Bohemian or Hungarian dynasty is Jagiellon
- Is year between 1500-1525
- Austria has set Poland as rival
- Austria is Emperor (maybe ?)
It cause Austrian events with options:
1) "Accept agreements with Jagiellons dynasty" with effects:
- Poland remove Austria from rival and Austria remove Poland from rival
- Polish-Austrian relations +100
- Austria dissolve alliances with all Poland rivals and vice versa
- Makes Bohemia lesser partner in union under Austria (if Bohemian ruler is from Jagiellon dynasty)
- Makes Hungary lesser partner in union under Austria (if Hungary ruler is from Jagiellon dynasty)
2) Decline offer from Jagiellons with effect +X prestige
It can be just, historical and give chance for:
- Bohemian and Hungary independece with quitenew line of history
- Bohemian-Hungary Union
- Austrian-Hungary-Bohemian Union
- Austrian-Hungary Union
- Austrian-Bohemian Union