Something I was considering for Just-Dice, but never actually implemented, was allowing people to "borrow" coins to invest. It would work like this:
All investments risk up to 0.5% per bet, but you are allowed to "borrow" up to N times the amount you have deposited (for some N, maybe 10).
Allowing people to borrow on margin seems like an interesting idea. It certainly would set dicebitco.in apart from the others.
Giving people the freedom to invest with a 10X Kelly seems like the way to go, but I fear that this could go bad so fast. It would "artificially" inflate the bankroll and max bet.
However, I wouldn't mind actually letting people really borrow from me for a percentage if the "auto-buy-to-cover" feature were enabled.