There are a set number of ways to spend family wealth, probably all detailed on the wiki. Knowing of them isn't a matter of creativity.
And what would you have people do, set huge bounties and fund revolts? Both of those options are ineffectual. Invest gold or ask for family support? Those do not spend wealth, they merely recirculate it. Buy regions or pay ransoms? Far too situational. There just are not many worthwhile things to spend money on. At least hoarding money gives you the satisfaction of hoarding it. Blasting it all on negligible effects is fleeting and insignificant.
If I were to really want to spend huge amounts of gold, I would simply build and then demolish recruitment centres over and over again. That would provide a chance at some notable gain, in the form of a rare, high quality recruitment centre. That is what I have been doing to try and spend all of my gold. But it has nothing to do with family gold, and it only cuts into excess gold, which is gold that is not very valuable. It is a 'meh' response, that is all.
Ten years ago I was a noob always trying to scrounge up enough gold to recruit a ten man unit, which would often be eliminated mere days after I bought it (battles were very frequent back then). That kind of challenge built in a sense of urgency and intimacy between myself and the wars I was fighting. It's often said that back then common nobles were merely pawns. From a grand strategic view, that is true, but from a more limited perspective, it is false. The personal barrier of trying to obtain gold back then made me far more engaged with the game. Nowadays I really do feel like just an automaton. There's no struggle in getting gold to recruit a unit. My only struggle is getting gold to build recruitment centres which I don't care about. Recruiting 800CS units is easy. It's no struggle at all!
That is getting a bit off track. There is a gold problem, and the Family Wealth thing is a direct result of it, but at this I doubt the former will be solved, so the only hope is to solve the latter, as the latter is very easy to solve. Either by deleting the cause of one of it's negative effects (the silly looking Top Families page), or by raising the cap, which is what this thread is about. Either option is easy to perform, so that is why I bring them up, rather than discuss the more difficult solutions to the matter of excess gold itself.