Instead of worrying about specific SEO methods, I would think about a general SEO principle of providing value.
Websites that provide value tend to more easily generate the quality signals that Google wants. For example in the best circumstances it is a challenge to develop backlinks. If your directory isn't providing a great value to users, that challenge to generate backlinks becomes even larger. Most directories do not provide great value and this is why many of them have greatly reduced rankings over the years.
Think about what great value your website can offer that is not already being provided by 100 other websites. If you can't answer that question you might want to rethink your business model because no matter what title tag or keyword density or siloing technique you utilize, you can have a very difficult time in surviving.
You are competing with thousands of other directories. Many directory links have been identified as unnatural links by Google, and because of this many people no longer look to directories as a source of links and traffic. In other words, you have a long and steep climb to make it to the top. Unless you can compete with the big guys, by investing a lot of money, time and moderation into your directory, my advice would be to pursue something else that has more of a future.
Are you in need of directories? Do you need sites to submit your site? Link building is not thing that you should consider for 2015. Rather you should contribute to the Internet by creating useful content. In this way people will link to your content and you website value will increase. Have a good time in the coming year.