I like the idea of a temporal storm specific hostile mob. There would have to be some way to prevent it from spawning inside your shelter though to incentivize taking shelter during the storm, such as not spawning at certain light levels, or within a certain radius of certain blocks/items (lanterns, perhaps). To build on this, I feel there could be another deep underground variant of this new being, or just a separate new underground being, because just having drifters and bugs underground doesn't make it too spooky/difficult.
Love the painting idea, there are already a variety of colors one can make from existing berries and flowers, and might encourage the addition of some new plants to fulfill this purpose. Adding a painting canvas and easel for small, portable artwork would be a good idea in my opinion as well, and perhaps using these pigments to write on signs and signed chests could replace using coal, chalk or lapis for this purpose.
I can imagine, spending hours in game, painting a masterpiece on canvas to hang up in my castle somewhere. Would be awesome.