I don't know if you're familiar with the Kecleon Shop in the Pokemon Mystery Dungeon series, but I think that's a good concept that could translate well.
Basically it's a chance for a floor to spawn with a big room with an outlined area in it.
In that area are randomly generated items, like weapons, tools, potions, etc. If you pick up (or use) an item from the shop and leave the area you're stopped and asked if you're going to pay for the items. If you choose yes the cost comes from your rupees. If no and you don't return it (or can't) then shopkeep enemies spawn across the floor and seek you out, so you have to defeat them or quickly escape that floor.
I think something like that would be a decent intro of a recurring npc plus a shop feature.
Also with your current gameplay I think it would be hilarious for a random enemy to unwittingly steal something from the shop and then be hunted down by the shopkeeps.
That sounds really interesting, I definitely want to add a more spelunky vibe to the shops at very least. I love the idea of monsters randomly walking in and stealing items but that's definitely something to think about later.
Could use some more player characters in the picture to show more player customization.
I realized this after making it, I also could have put the skeleton in a hit reel animation with pieces coming off of him. I just didn't want to over complicate the picture, or spend too much time on it since I have a lot of stuff to do.
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I've been looking at the sword slash effect, trying to add some meat to it,

If you guys know of any cool weapons in blockland or any game that have a neat trail effect to them I'd love to see pics, I need to find something I really love for this effect.