Making a plugin that allows players to level up by experience points. Then let the server owner adjust how much experience is needed to reach a max level, and even set the max level in a .yml file. And the only way to get experience is by killing mobs, making Minecrafts replay value go sky rocketing by giving players a real purpose in the game, something to look forward to. The higher the level the stronger you become, or at least the more HP you get. It reminds you of an RPG system but just with the leveling and experience part of it. The higher the level the more equipped you are for PvP battles. I can't even tell you how fun this would be, you just need to experience it. The first developer to make a plugin like this I will EASILY donate $120 to you because it matters that much. This would also give you purpose for killing mobs. Everyone wants to be "the strongest" and this will give players hard work something worthwhile.