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The reason the gen 1-5 box legendaries suck (except for the weather setters) is they don't fucking DO anything, yeah they have good stats but they aren't that much more powerful than the average 600 bst mon because they aren't minmaxed and they have niche abilities like Pressure or Mold Breaker reskins. Maybe you'll get a decent signature move or you'll be stuck with a meme that's barely better than an existing one.
Modern legendaries actually DO things other than having bigger numbers, they can sweep you, support the team with field effects, or spam a really powerful move.
First step to fixing the old legendaries is strip all of the pressureslop, give them abilities that affect the course of a battle, and make slight minmaxes to stats they don't use. As long as things like water spout whale, ice/ghost horse spam, and electro lizard exist there is little reason to use most of the box legendaries in the game.