1) Allow orbs to drop and in larger amounts than Challenge mode
2) Add celestial evos
3) Add greater materials to the material chests
4) Make the Boss Difficulty Boss Mode levels cost 20 stamina, have them drop superior materials and runes.
The above alone would make them worthwhile, but consider also adding:
1) A new hero, as was introduced in Challenge Mode, that could only be starred by completing Boss Mode. Make this hero a PvE powerhouse, but vulnerable in PvP.
2) Daily Boss Mode quests that award materials and tokens toward the Boss Mode exclusive hero.
3) Add titles for completing and 3 starring Boss Mode levels
If there are any other ideas, post them here! What could be added to Boss Mode to coax you to play levels outside of Boss Island?