not necessarily SaaS products, but I'm thinking of software for professionals:

- software for lawyers, accountants

- software for the health industry

- graphic software such as photoshop, 3DSMax or Final Cut pro

the overall market is smaller, but you can sell the software for more. Also might need a little bit more topical knowledge, but imho it's the same quantity of work as doing a Microsoft Office clone

another market with fat margins is MMO video games. I'm sure there are lots of niche markets out there wanting their own game