For new assets, maybe a spell effects kit, more animations for characters, or possibly consumables. I think they will all fit nicely with what you have already created.
Either way I like your art, it is very simple, but effective.
Here is you an update where I am a now. https://www.reddit.com/r/godot/comments/plkimr/update_on_my_little_topdown_game_...
A few more suggestions, is maybe some houses, cave tilesets, and dungeon tilesets.