Hi again!
Thanks for the compliment on the art ;)
Passive items are something that I've wanted to add for a while, so I certainly plan on implementing them in a future update. For now, I'm going to work on fleshing out the upgrade system, and then I'll get started on passive items. If you have any ideas for passive items, then feel free to let me know. I can't promise anything, but ideas are always helpful and inspiring!