Allow us to despawn our mercenary without losing gear or paying their fee again. There are situations that just 1-shot mercenaries with no hope of them surviving and I know they will die, so I don't want to bring them, however, dismissing them preemptively still just costs me gold. Additionally, sometimes it's very hard to test stuff on training dummies with the merc there.
Obviously I would also like it if things didn't guarantee kill mercs, such as the Act 5 boss dropping balls of doom on his head and then just watching him swim in them till he dies.