Burn the Obami's family to the ground, is my theory.
i feel like her motif is simple and is well established at this point: she just wanna gamble and she wants everyone to ennjoy it as sshe does
That's a nice guess
I mean that isn't a bad Theory since one of the recent chapters, forgot which it said, "Even though it was Kirari who killed Jabami" that HAS to be related to Yumeko's sister since she's the only other Jabami we know. But Yumeko isn't the type to hold a grudge nor did she seem to care when her sister had pretty much lost her whole life in a gamble. But then again that's all Anime Only so we don't know when it comes to the manga
My guess is that Yumeko actually has no attention on being the president, she actually want something but no one knows what it is