Not sure how much you will make on this, but once you design the thing you need to sell, it immediately turns into a marketing effort in order to get designers to buy it. Designing the UI kit/template, etc is not the hard part - it's convincing designers that your solution is the thing they should spend their money on. I'd say the most well-known (or at least prolific) is the Untitled UI guy. He posts a lot, on a lot of platforms. I'd take some inspiration because I see their work everywhere. And the kit is pretty good too, from what i can tell. Answer from chillpalchill on reddit.com