I was actually just thinking about this today. Great suggestions. I really need to get some sleep for tomorrow. I have a bunch I want. I will post soon I promise. Been stuck working through bugs and customer service/in game help.
But I don't want to go too far off track so here are some real quick
Have 10 armies on the world map
Acquire 25000/50000 (resource)
Acquire 100k gold
Kill 100/1000/10000 units during PVP
Successfully siege 10/25/50/100 players.
Successfully complete 10/50/100 co-op siege battles
And to elaborate on this, have achievements award crowns/resources/maybe elite units or special units depending on difficulty.