Will SCOTUS overturn affirmative action?
Will SCOTUS rule against unions?
Because the justices of SCOTUS rule more after political leaning than after the law, the decision of the court are legitimate targets of betting. Furthermore, the decisions are the most powerful orders our system can experience, stronger than a law (which could be overturned by SCOTUS) or a presidential order (which each court could overturn).
This is a good idea! There could be markets for individual cases, like "will SCOTUS keep/overturn this law". There could also be margin markets for what the vote will be, such as 5-4, 6-3, etc. There could even be markets for how individual justices will vote.