You can add even more achievements with smaller prize.
Try to focus on something that is not too hard for casual players, e.g. raid x resources, something with bgs e.t.c.
If you give some legendaries as a reward, you may promote it to players who don't buy.
Beat lvl 10 off bg - reward 1 standard bearer
Beat lvl 20 off bg - reward 1 twinblade
Beat lvl 30 off bg - reward 1 banshee
Beat lvl 40 off bg - reward 1 hydra
the same for def bgs, only double the rewards (def legendaries)
It's probably too late to add more levels to existing achievements, except maybe produced ress and few more.
I want to suggest the following individual achievements:
- reach Fireborn level 2,4,6,8,10
- activate fireborn status 10,50,100,200,500,1000...times
- exchange magic 5k,10k,50k,100k,200k,500k...
- hire hexbeards 1,3,5,7,9,11
- kill legendary units 10,50,100,200,500...
- combine runes level 2,4,6,8,10
- complete saga quests level 5,10,15,20,25...
- upgrade keep/stroghold level 5,10,15,20
- reach heros development level 20,30,40,50...
- claim gift from King Bjorn 50,100,200,500,1000...times
- win stronghold battles by defending 10,50,100,200,500...
- collect resources from different settlements 5,10,20,50,100...
- use skill points 10, 20, 50, 100...