I'm looking for new software to play around with, even though I love MyPaint. I've found that Krita is available from the Software Centre but I'm still looking for more options. I've been wanting to try out Paint Tool SAI, but haven't really bothered because it's only a trial and wasn't sure on how to install it in Ubuntu.
My favourite would be Pinta Image Editor. More details you can find on their website.
Of course, GIMP too. Hope you like it.
Try Krita, it has a lot of templates to choose from and is easy to use. This can be found in the software centre. https://www.calligra.org/krita/