If you've already done lots of work and link building on your old directory then it would probably rank quicker adding onto it instead of starting a new one. However, starting a new one would give you a better chance to make it focus solely around that individual topic and it might rank for more similar keyphrases.

If you plan on spending time with it.Adding content..building links..marketing it, then it might be worth starting a new one. However, if this is something you don't want to spend allot of time on then just add it to what you have already, it will require less work.

If you are willing to give time and effort to the new one then i will recommend you to build a new site because it will be a fresh start and more specific to your niche which will get quite easily ranked.

Hmm.. if it is relevant to your current niche then you can just add it to your existing site, however if you want to focus on it mainly then just build a new website out of it.

I suggest you to create a new directory if the niche of new one is not relevant to the old one.
It's better to not build one site around topics in different categories. That may hurt your current ranking.