Infernum/Eidolon reaper
I think a boss with a exclusive drop would be something needed in the current state of the game, there are to many "easy" ways to get everything you need out of boxes or farm without much fight for it..
"Eidolon Reaper" was the best thing Justac had in years due to its absolute necessity for specific items/title.
Even if the map around it had alot of bugs and flaws, it was something created for fights with high reward of drop.
Similar to what we currently have with Ancient, but the map is to small and can be cleared by a single player in less then 5 mins. Then you have to wait, Infernum had no wait and constant farm/fight.
I hope to see something similar in the futur.
Bit 49-79
I dont think there are anything that would make the world "appealing" for anyone to go there, you can already get nectars from Ancient and Asthal-Scion from Boxes.
It would need a whole new purpose, but similar to "Battle Square" the elite gear ruin the whole fun that world brings.
It would need to be a world only "realm lv1" could be used as gear to make it "fair" for everyone.
Lets see what the futur bring !