Well, several things can happen ...
1. Firms and exchanges immediately react to the law and change themselves, or create new entities, to trade, under the gaming industry protocol. Afterall, the truth is out - it is gambling
2. DJIA drops 10 to 25% of its face value on the day the law kicks in, then spiral down to almost nothing. Why? Valuation of all long term pension/mutual/trust funds must be discounted by 1/2% immediately as all selling of assets have the added cost of 1/2% ... the impact gets infinitely larger as each transaction these funds have to carry out are taxed. Fun to see these funds go down in value every month even when the market is doing absolutely nothing.
3. Widen spread - bid 1% below and ask 1% above, across all taxed instruments.
It is fun to see how a government can talk out of the mess of this scale due to the introduction of a law with "good intention" ...
Edit: More I can think of here,
4. A drop of 50% to 75% daily volume is a sure thing - kiss of death for most brokerage firms.
5. All normal trading businesses and hedge fund businesses will go to Europe and Asia.