I think it would be nice to have some early exp(and maybe even money) upgrades that require money. Because as it stands now you don't care about your money at all in the early/mid game. It's all about getting exp by playing more hands - these allow you to get all the winnings/losing money upgrades and from then on you snowball to a billion.
I know that's partially a "ruining the game by optimising" problem but i still think there should be some more incentive to make money early.