The seal of the U.S. Securities and Exchange Commission hangs on the wall at SEC headquarters in Washington.

Today is the final day for the trading industry and investors to submit comments on one of Securities and Exchange Commission Chair Gary Gensler's most controversial proposals: a partial overhaul of the U.S. trading system.

The package is so complex and potentially far-reaching that Gensler has broken them into four separate proposals. Two of them have sizeable industry support and are likely to be adopted. But two others are facing serious opposition and, at least one — the effort to replace payment for order flow by an auction process which came out of the GameStop meme stock controversy — may end in litigation to prevent it from seeing the light of day.