>>57751940I feel like I'm onto something.
Rather than painting the tileset by hand like a madman, I may be able to pull off automating a good part of it with Photoshop. For this test I took part of a HGSS tileset and did the following:
1) Increased scale 200%, bringing each up tile from 16x16 to 32x32
2) Refined the edges the clone brush, enhancing the contours with more detailed curves
3) Applied Filter > Paint Daubs with Brush Size 1, Sharpness 0 and the Wide Blurry Setting
"Enhanced" on the left, original on the right. Exported at 200% for a more accurate picture of what it'll look like in-game.
The filter leaves a few artifacts that I'll have to manually airbrush out and I'll refine some details but I find the overall effect quite satisfactory at creating a more painterly tile.