Actually, I think the ideal law would be something to the effect of "it's illegal to IDB at all, except in weight/balance issues on small aircraft where an extenuating circumstance requires the aircraft to fly with some number of empty seats." In any other industry, we'd call it fraud. Sports teams and theater companies don't sell 40,000 confirmed tickets if they know the venue only holds 30,000 people.

In reality, I know that won't happen, so the alternative is to simply make it incredibly financially painful for the airline to commit this act. There should be a meaningful minimum - say, $1000 for a 2-4 hour delay and $3000 per day for an overnight or longer delay. I'm less concerned about a cap: establishing a floor with some teeth is much more impactful. (It would be a very rare flight that sub-$1000 wouldn't generate volunteers on a short delay, for example.) But thinking of the F passenger that United threatened to handcuff - a guy on a paid F ticket - maybe there shouldn't be a cap and it should be a multiplier of the ticket value.

I don't think VDB needs *much* regulation - perhaps some basic rules to prohibit restrictions on how airline vouchers are used. I think it should be fair game for passengers to bid for miles, upgrades, or vouchers as part of a VDB process, but tricks like single-passenger, single-use, expiry dates, nontransferable, limited to certain fares, etc. should be banned. Those rules are never explained at all by gate agents: they always make vouchers sound cashlike towards any tickets purchased in the future.