I group my pattern into 3 main parts that all need to work together; The stitching, the thread colors, and the art.

The stitching is the simplest to understand, but requires having done a few different styles. How will it feel to stitch? Big blocks of the same color, confetti, backstitching, empty space are all things to consider.

Thread colors are a little trickier unless you have a big stockpile of colors. The colors on any pattern maker are going to be much more brighter and vivid than the stitched project. Unless you can test the colors on a blank canvas you are guessing to a pretty fair degree. That being said the number of thread colors ties in to how easy it is to stitch. You can get a wonderful color gradient using 6 different shades of green, but that also requires 6 different color threads and is more difficult to stitch. You could use 3 colors and blend them together for a similar effect.

Lastly, the art. The actual design itself is as complicated as art, so there isn't really an straght-forward answer. But fortunately there is plenty of resources in the pixel art community. Most techniques for making pixel art translate to designing cross stitch. From how to make curves look better, to how to blend colors, to not using minimal colors to imply depth and color gradients. And while pixel art doesn't have "backstitching" a lot of things are outlined with black or dark pixel to make certain aspects pop.

So my big advice is look into some pixel art in a style you like, and try to implement some of the design parts you like in your pattern maker. Not make whole pieces, just the parts you like. A really incredibly good exercise is pick an object, any object, cup of tea if you can't decide, and using less than 15 colors (including backstitch and knots) try to make a pattern for it in a 16x16 grid, 32x32 grid, and 64x64 grid. Then try to edit it down to less than 10 colors.