A good update would be a Grid enlargment upgrade system
I understand that this isn't super easy to add, but I don't think it would be taking that much time away from you if you work on it at your own pace.
You can also add more "units" I don't have any ideas for types to add, but you can always make a poll list for people to vote here and on social medias, this small game can become larger. It's popularity depends on your effort.