I have a few suggestions related to custom art.
When you make custom art, you should be able to have a hsb or rgb colour picker. That way, you could include lots of different colours, like a colour wheel.
Custom pen sizes
This would be similar to the previous suggestion, but with pen sizes. At the moment, there are only three sizes, and they are all pretty similar. There would be a slider or something similar to adjust the pen sizes.
This would make drawing outlines easier.
a fill option
This would fill the inside of a pen outline, making it look less messy than having to colour in the inside.
Giving non-subscribers some free custom art.
This is pretty self-explanatory , maybe give non-subscribers like one free image per month.