be able to play a child other than the main heir

[Keliev]

I swear, ck3 is so easier than CK2. To increase the difficulty :
I would like to be able to play my second son (or third) each time.
While leaving the inheritance to my eldest

 

[Matihood1]

You can already do it with "Switch character" button from the Escape menu, if you're playing ironman.

 

[Chatoustikmou]

There are also several mods that do this. In the one I made, Inherichance, you decide with a game rule which setting you want: play as a random heir, play as the weakest heir (i.e. the one that inherited the smallest domain) or play as the strongest heir.

It's arguably better than the Switch Character vanilla feature, because if you go for a random heir, the fact that you don't get to chose changes how you plan succession. Plus it's seamlessly integrated in the Succession and Lineage windows, which is nice.