Optional 2-of-2 multisig trades
Give us 2-of-2 multisig as an option for trades.

Besides the actual XMR being traded, Alice and Bob each additionally lock up an amount of XMR equal to or greater than the value of the trade in a 2-of-2 multisig. Whichever side the scammer is on will always end up losing more than if they would have cooperated.

Not 100% guaranteed (nothing is), I know, but both sides are strongly incentivized to cooperate if they're economically rational. Vast majority of users, and even would-be scammers, are.

Pros: Doesn't potentially rely on a third party (an arbitrator), that might be in collusion, or require trusting their correct judgement in arbitrating
Cons: Have to lock up a larger proportion of XMR for trades

Unless there are good reasons this hasn't been implemented yet as an option that I just haven't thought of