Everyone has a slightly different approach. I recommend you write 10 hubs on a range of different topics and see which do best before proceeding with a longer-term plan. The reason I say this is that hubs of mine which I've thought no-one would visit (I wrote them for enjoyment), have actually had pretty good traffic. Other hubs I've made that I thought would be waay popular... were just not!
In other words, it's the writing and the way the topic is handled that has the impact... some people seem to have more success with certain topics, others with other topics. It's definitely worth branching out a bit so you can start learning now what works well for you (unless for some reason you are purposefully trying to stay tightly focused in one specialty area). Well, that's just my opinion... hopefully others will have other ideas for you.