Noted. The full game will have you start with "junk" items in your deck, so I could add in an effect to remove/transform those.
And there's a few effects that let you alter your cards' cost, though maybe I should add more.
I would say pay to upgrade is fine. If you were super fancy you could combine items into others, but that sounds like work.
A few more one cost items would be helpful, especially a one-cost item that let you use it multiple times in a turn to poke so you didn’t need to buy multiple. I found myself wanting to poke the enemy a lot because I had one more power but nothing to do with it on probably about 1/3 turns.
Edit: Something like Basic Jab: spend an energy to poke a single world for one damage (option always available). Useful enough to disrupt certain effect cards, lets you burn extra energy for something. Would be less useful as more worlds, but hey.