Wallpaper's really taste-dependent, everyone's varies. Wallpaper suitability also varies by the resolution a person runs their MiSTer at, and can take significant storage space if nice clean PNGs are used.
That said, the MiSTer's built-in wallpaper choices are spartan to the extreme. Maybe a small (relatively neutral in content?) wallpaper starter pack+ could be nice to have available.
The revamped MiSTer Downloader script lets you easily add additional archives to sync to; projects like
MiSTer BGM and
Super Attract Mode make use of this to allow effortless in-place updates. Someone could create a nice wallpaper project on Github and allow users to automatically pull any updates in a similar fashion.
I actually made a collection of wallpapers in three resolutions (1080p, iPad, some other res) some time ago that I've been meaning to share here but haven't gotten around to packaging it yet. (Also the HDD it was on just died Saturday but that's another story, lol.)