No random succession!
One thing in the game are really fastidious, the random successions.
I hope for have an accurate system for this mechanics.

I thought that:

Royal marriage cost 100 diplomatic points.
If you have an alliance with the other country you have an -50% of the price
If you have an long time alliance ( 50 years ) there is an other -10%
Different religion give a malus, same group ( for example christian group with catholic, protestant, reformed, coptic and orthodox ) give you +30% of price. Different group give you + 150%
At the last the price is multiplied by ( 200/relation value )

But this must be applied on an genealogical system. In the royal family you have father, mother and children. Different form of government establish the right of succession ( only the eldest son, only a man, only the eldest man or woman, one chosen by the king, ecc.. ec.. )

A country have priority on an other by different things like diplomat reputation, the longest time of the alliance, army strength, common rivals, ecc... ecc...

In this way, for example, if i play with the Hannover and i do an royal marriage with the UK, the usband is my heir and the wife is the only daughter of the king of UK the first son of this union is the heir of Hannover and UK. You can choose if do an personal union, but can born an coalition to delete this ( like the Spanish Succession ) or place another son on the throne of the UK.

Other things are:
An king, old ( 60 years ) or without a wife, can't conceive an heir

If there are two single rulers, an king and an queen, with the royal marriage we have an automatic personal union, but this marriage have a base cost of 400 diplomatic point

This is an idea to use like an example.