If you ever get such ridiculous margin you should buy things that have high chances of very small profit, such as selling deep OTM options or credit spreads. Wait for expiration, unfold the scheme, collect your profits, disappear.
If you leverage up to $1M a small 2% profit with 95% chances of success is an easy thing to achieve by selling options, and would pay you $20k instantly. In the 5% outcome your margin will just totally blow up, like LTCM in 2000, or the whole idiotic concept of Value-at-Risk in 2008.