All market participants must have margin available to clear all orders they have open at any instant in time on an unaggregated basis. Since you may only enter an order you intend to execute under black-letter law you must be required to have the margin capacity through either cash or secured and proved-available credit, to clear the trade. Period.
All orders must be exposed to actual execution risk by all market participants. Since you may only enter an order that you intend to execute the market must be able to act on each and every order you place into the market. This requires that each order, once placed, by valid for a reasonable minimum period of time so that it is exposed to a a reasonably-large percentage (for all purposes all) of the market. This means that the minimum human reaction time plus the round-trip time for all reasonable technologies in use must be the minimum order validity time; an order must either be valid for that time or it must execute. A reasonable definition of this time is 2 seconds.
End of HFT advantage through quote stuffing. End of Flash crashes. What HFT has done is increased the velocity of the market and basically allows for a normal years worth of human trades in about 2 weeks, skimming pennies and fractions of pennies through that quote stuffing over that time frame at blistering speeds. This is blatantly illegal conduct which the SEC just over looks.