I hope there are none or few event legendary Pokemon in the next games. Gamefreak went overboard making event legends and legends in general during Gen IV, making each legendary Pokemon seem less special. Also, few players could actually get all the event legendary Pokemon.
Instead of event legendaries, there could be an extremely difficult dungeon with a legendary Pokemon at the end of it. The dungeon would be so hard that a regular player could not get to the end, except with all level 100s. Such a dungeon might, for example, require all the HMs, have wild Pokemon with levels in the 80s and 90s, contain trainers with level 100s, and be very long. Having a dungeon like this would make it so the secret legendary Pokemon would still be hard to get, but any player with enough knowledge of and interest in the game could get it.