Hello since for johto we have the ho-oh rematchs that gives x10 rainbow qwills and x3 legendary lure, id would suggest to give an addiction of x10 elemental orbs , (as counting kanto and johto a unique one, because there are no other legendaries in gen 1 aside of mewtwo that you can find it in the wild for war battles to contest it.)
Also if possible for the future would be nice to add legendary battles/rebattles in other regions for example:
Hoenn: a battle against kyogre and groudon. that can give still x3 legendary lures and x10 of the future items to increase/decrease ivs of the future regis/duolati that will come.
Sinnoh: Giratina rematch in his world? that can give the lures and the future items for ivs for the trio lakes?
Unova: in the giant chasm there's a locked zone that you could find kyurem, would be amazing if that spot one day would be opened and battling the kyurem with the same rewards as i said before
for the trio sword masters or the genius.
Ideas for item ivs set up for legendaries:
Kanto/Johto: Rainbow Qwills/Elemental orbs.
Hoenn: Dew Drops/Eon Tears if duo latis.
ancient rocks (if trio regis)
Sinnoh: Lakes psychic orbs (?)
Unova: Pieces of swords (for the trio sword masters)
having more legendaries battles/rebattles would be a nice weekly grind.
Guess you staff are enough smart to find out a way how to make em as battle.
sorry for my english.