I am creating a free SEO service and part of it is going to be a directory. I am looking for people to list some niches since I am sure I have forgotten some.
I'm sure you've already reviewed your competition as part of your business planning. Why not just learn from them? Also remember that lots of empty categories is off putting so be prepared to start small and expand as time goes on. Takes a bit of effort as you have to move existing listings and possibly tweak their titles but it is worth it. I used to have a real estate directory that started out with every USA state, but as the listings grew I needed to create cities and sometimes even suburbs. Once that was done having a title saying "Denver Real Estate" looked downright spammy so I'd visit the site and find out who owned the site and change it to "XYZ Realty". Took work, and maybe meant they weren't getting the keywords they were after but the page was highly targetted so that would have compensated.