The reason I don't chase mementos is because I can only have one at a time, and I don't necessarily want to go through all my crafted weapons and decide which one looks best with the memento at any given time. Even if it's just 3-5, being able to stack a few of the same memento would be very nice, and would make me much more likely to grind them out.
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Wish we’d have more than just the 3 in game and event based ones, master raids/dungeon mementos or other activities would be nice too.