Before you start chasing new keywords, make sure that they are relevant and useful keywords for your customers. Remember that the purpose is always to bring lots of the right traffic to your site, not just lots of traffic.
So before you do anything, check that all that great traffic you're getting now is actually useful to you. Is it converting to sales, if that's what you ultimately want? If you haven't got goals set up already in google analytics, go do it now.
Assuming that the traffic really is beneficial to you - ie visitors are completing your goals - then analyse where the traffic is coming from, and on what keywords. You might find that traffic from a certain website brings in higher value visitors than organic search, for example - or vice versa.
Ok so now you know you're getting loads of traffic, it's converting well, and you know exactly what types of traffic converts best. So you want to replicate that and get even more traffic.
Maybe all the traffic and goals are around one product or section of your website? If there is content that no one is finding, that's what you should concentrate on. There's no point in ranking well for keywords that have nothing to do with your website (plus it's really hard to do). So find that area that is getting ignored, and create some great additional content on it. Blog posts, infographics, beautiful photography.... whatever is right for your content.
Now go back to your analytics, and see what you can learn from the bits of your website that already do well. Go to websites that are linking to that content, and see if they might want to link to your new content. Getting loads of interest from social media? Seed your new content there too. What you're trying to do is replicate the success you already have.
Ultimately what you need is good quality links from good quality websites, pointing to your new content. This will do wonders for your SEO and you'll start getting great organic keyword traffic.