For me it would be a system that allows players to create their own magic(like in "Two Worlds 2) or combine seperate magic spells into one unique spell. A summon system where are given the OPTION to create /customize our own summons with different body parts and the power of said summon depending on the rarity of each body part. Combos for meele bound classes.
Also, a title + tournament system where players fight other players from their specific class archetype. The winner of the tournament for that specific class archetype gains a title along the lines of " Greatest Blade dancer" for example; with said title giving players who obtain it a 5 - 10% buff in all stats relating to the class OR a powerful unique skill(that's limited to only the bearer of the title) when fighting in pvp(and maybe PVE) and if they let's say loose in a pvp against a corrupted player the title is taken away. There could also be a tournament between the winners of each class archetype for a much higher title(with much greater stats buff) "Strongest being" for example.
There could also be a tournament between servers for the winners of each tournament for an even greater title and stats benefits or something else.
Update: There could also be a betting system for non-participants of the tournament.
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