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So casual (lack of updates/content) or repetitive grind of same content is not what I'm looking for at end game.
And by frequent I mean within 1-3 months period.
Eh... Nothing really comes to mind, but Grand Chase/Elsword (both of which are 2d beat-em-up style games) get rather frequent additions. Whenever I played Grandchase (which was usually every 2-4months for a month of time+) There was always a new character/new job for a character getting hyped/released. Elsword looks like it may follow a similar procedure.
But hey, they might not be your type of game, just though I'd throw em' out there.
I wish you luck in your endeavors.