Honest to goodness, for affiliates, I sincerely feel like the online gambling vertical can be an incredibly lucrative arena.
With that being said, there are still other very profitable verticals. For US traffic, given the current events & economic state, plus what are trustworthy niches that have been strongly established, I would suggest the following:
1) Education
2) Home-based Business Opportunities
3) Financial: Credit Cards, Credit Reports & Debt
If you’re able to find lead-gen campaigns in any of these verticals, go for it. Best of luck to you!