-Preserve the rarity of the Elite Four
-Provide a better way to train up your higher level Pokemon, without easy-moding it (strongholds are super rare, after all)
-Add a cool mechanic to the game
Details: The 4 challengers would spawn randomly inside the stronghold, but only 1 of each can and will spawn. The champion could be at the end portal? Just an idea. Of course, to be in line with the games, all of these trainers would need to be relatively challenging and have good teams of Pokemon. There could be rewards similar to killing a boss? Also, I think the Elite Four should be able to be challenged more than once, but maybe have a time limit. There are so many things you could do with this; take away whatever you want from this suggestion.
This is my two cents; I think it would be wonderful to see in-game. Good day!