rmnr You are off to a great start. As others stated, consistency is definitely the number one priority. I think finding your own unique style is really important, too. The icon design business is so competitive, and in many ways over-saturated, I think having a recognizable style with your own interpretation, without going to far out, is important. I also think it is good to specialize, too. Expanding on some core niches can be very fruitful.
For instance, Gan Khoon Lay (https://www.iconfinder.com/leremy) does only the abstract people doing different activities. But he has thousands and thousands of them. He told me that he spent over 5 years creating them but they sell EXTREMELY well. He is a top seller on every marketplace where he sells including on Iconfinder. He even told me once, "I know the designs are not the best, but I have nearly every activity anyone could think of" and he is 100% correct. What this demonstrates is that consistency and thoroughness are very strong selling points for customers.
I recently started trying to explore specific themes with my icons. I have a collection of over 800 avatars and also started a "Hand Gestures" collection that I plan to expand to several hundred icons.
A good way to choose is to check out the latest Icon Designers' Report that Monica puts together every quarter for Iconfinder. It is a treasure trove of useful info. If you pick some of the most searched-for keywords and categories and focus on expanding on those way beyond what is available, it can be very profitable.
Typically, designers will create 50-100 icons in a particular category. But there is always a need for obscure concepts and combos. You can also focus on things like "Air Travel", "Plumbing", "Blueprints", etc. There are still vast untapped markets out there. The key is just finding them. A good way to do this is to stay current on overall search trends on Google, especially related to UI design, retail sails, technology trends, and marketing. People will buy whatever is currently in high demand in the retail markets.
The main reason Streamline Icons is so successful is that Vincent was very savvy and recognized when Apple changed from Skeumorphic to the line icons there would be a huge demand. He was not wrong and built a very successful business off of that one brilliant insight.