Yes particles would make it better, specifically dynamic particles or random particles, like angled particles or some that follow the dragon, or that just appear when you do something, it makes the player think he has more control and feels more satisfying. The particles when you do something is also known as player feedback and is one of the three most important things when making a game fun. The others are multiplayer which in this case can't be added since it would be to hard for the jam, and the last one is randomness. If you make a game and it sucks, and you add those three things, or just 2 of them everywhere, I can guarantee you it will be fun. Also the palette in itself is fine but you should stick to 2 colours that are opposite or almost opposite and the shades of those colours, take an example from undertale or hollow knight. Also like you mentioned the art could be better. I recommend looking at some videos from Jonas Tyroller on YouTube, so you can see what I mean by player feedback, his tutorials are amazing. Anyways I hope this helps!
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Funny that you mentioned it, since I watched entire Jonas, multiple times.
I feel like 3 colors is not too much, it's not like I am using 7 random colors, maybe those colors are not complementary (I didn't check), but I like how they look togetger, and I chose other colors to fit those 3 main colors.
I have never worked with particles (I don't think it would be hard), honestly, can't imagine where would they fit, it's not like birds leave trail where they fly, maybe I could add them to coins or cities, but I don't think that would make a big difference, at best I could make something like explosion whenever dragon robs cities.
I couldn't make better art because I completely exhausted my human limit, as I said before, there were many ideas I wanted to implement, many things I wanted to fix, actually, I wanted to enter WeeklyGameJam, but, it does no longer exist, for some reason, I could only do that much in 2 days, and I also felt ill on 3rd day.