Mission modifiers you can set before you start the mission and get very small, stackable (additive) bonuses on gold/souls
You'd have to unlock each modifier from quests or leveling up
Some ideas:
-More enemies
-Only elite enemies
-More enemy health (would not apply to xp)
-Faster enemies or enemy projectiles
-Tunnel vision
-Lower health pool
-Party mode (all enemies have a party hat, wouldn't give bonuses)
Theres many more ideas this community can come up with, this is just mine