Character unlocks are cool yeah. In games like this I tend to be intrinsically motivated to explore the dungeon and get as far as possible. An end goal could be good, and maybe optional objectives during the run. For example, a magic penny could spawn that can be thrown for a small amount of damage, or maybe it has a charm effect. But if you throw that penny in a fountain that might appear, that unlocks something or gives you a boon. Idk tho just a random idea. I don't think you should compromise the simplicity/accessibility of the game in service of new mechanics unless they are significant improvements.
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you have some good points the penny/fountain idea wouldnt be that hard to implement, i can already picture the logic in my head. you are also right about not compromising the simplicity of the game thats why i though a rpg leveling system would be a good fit as i doubt anyone would have an trouble understanding it