You could easily start doing recipes based off online recipes, or those from a book like "Brewing Classic Styles," or, "Clone Brews." There are plenty on this site as well. There is nothing wrong with a kit brew, they make buying materials easy, and you don't need storage for leftover ingredients afterward. But learning how to make your own recipe based on what you want to drink ... extremely rewarding.
As for bumping up difficulty or your system, you could do a few things. Perhaps try partial-mash like Unionrdr recommends. A lot of folks have been happy with brew in a bag (BIAB) as a step up, since it introduces them to full-grain. Full-grain brewing equipment could be purchased as well.
You could also approach things to help your brews become more consistent, like temp controlled fermentation, the equipment to make yeast starters from liquid yeast, a wort chiller, an aeration system, etc. And further you could approach things to help you store/serve/carbonate more easily, like a keg system.
For what it's worth, keep browsing here, and pick up a book or two on some more advanced brewing techniques (or check out the videos of the same.) You'll get more ideas about which thing you want to do first.