use of leverage is indeed very useful, but i think it doesn't belong in the protocol. it is trivial for sites to allow their own built in leverage. for example a fiat exchange can allow you to buy 10x bitcoins and only require you to hold 10% funds as collateral for margin. for your gambling example, https://dicenow.com already has investor selectable leverage from 0.25% to 30% risk per bet.

it doesn't make sense to add this to the protocol, because there are too many variables unique to each use case. how much leverage to use? what are the situations in which the site can perform a margin call and force liquidate the user? these are best left to each specific application's use of bitcoin.

also, you have to trust the operator 100% regardless, because they need the power to liquidate your position at any time (exchange: due to sharp movements in in price) (gambling: whale won, you lost) and take your coins. there is no purpose in setting up some mutual trust account since alice requires complete control over bob's onsite coins. it would only accomplish to show that bob isn't using dangerous amounts of leverage on alice's site, and actually has coins in reserve. does that really matter though? if bob wants to take ridiculous risks, let him. everyone else can keep their unleveraged coins safe in a cold wallet, or some other productive use generating returns (instead of frozen)