An exchange that doesn't close shop on sudden market moves would be a good start. (Coinbase, which you mentioned specifically, was unreachable the whole time.) One that would honor all orders, in the right order, and with enough liquidity to make customers whole when they for some reason didn't, would be even better.

Then some proper options would be nice. Preferrably someone who doesn't trade against their own customers, which at least one exchange is open about. There's a lot that could be done in the Bitcoin ecosysten (well, at least we are rid of MtGox).