Guild Wars 2

I saw this one working well on a mobile phone game called Arel Wars

There are bonus objectives (which grant you a nice reward) where you have to stay alive and defend your tower for about 5 minutes before you defeat the enemy. The challenging part here: don’t rush the enemy and kill him before 5 minutes.

1st suggestion: introduce events where killing the mobs as fast as possible isn’t the goal but instead makes it more difficult to win.

how to do this: exactly like in this one event in Orr where you had to enrage a norn-woman by hitting enemies long enough so that those change their state and then attack the norn.

why? because it’s a variation from all the mindless dps-races.

2nd suggestion: Introduce rewards for events for certain bonus objectives which are much harder to fulfill. Grant for example a reward like a chance to get a rare/exotic. This would make events much more appealing and challenging imho.

why? it is a HUGE incentive for people to finish an event (and not only tag it and run away)

3rd suggestion: give us specific rewards for challenges.
-) rare harvesting tools
-) rare bags
-) nice skins (I loved the headband which you could achieve from repairing a Charr-vehicle) something like that.
-) rare gem drops (having 300 gems in your inventory makes it more likely that someone buys the missing 200 gems to buy something cool for 500 gems)
-) lore-pieces (like pages of books, little stories etc.)