Wars for money tend to create feedback loops. Either you make more money than you spend fighting the war or you don't. If not, such wars are a waste of time. If wars are profitable, then you will tend to get into a positive feedback loop, waging war for money and using the money to support your army. That is also going to create balance issues for the warmongering trait.
It is already possible to use your army to farm piety and prestige. Attacking hostile but non-enemy armies gives you prestige, and if they religiously hostile evil, it also gives you piety.
Wars for releasing family members would make sense.
Dynastic renown is based on your family spreading through multiple realms. If your family is limited to you vassals in your own empire, your renown gain is supposed to be low.
Converting a ruler by a war would probably be OP, and even in the hands of the AI, it could generate silliness. In the earlier patches, it was possible to force a ruler a convert in order to let them out of prison, but that was patched out because it was OP. The entire Byzantine Empire shouldn't go Muslim or Jewish simply due to losing one war. Also, the would run the risk of forced-converted realms spreading their faith further. Aka, the Umayyads converting Aquitaine to Muwalladism, which converts West Francia, which converts East Francia, which converts Denmark, while the Kazars convert the Abbasids to Karabite Judaism, starting of chain of conversion heading toward India.
