I think shields should be in Xsyon.
Giving the player the ability to have a shield will add various forms of depth to combat. Base it off of the block skill. Have it add a slight defense bonus, but be heavy, and only really add its benefits when someone is actually clicking the "block" buttons.
This would add to players investing in the block skill as well as various types of craftable items. Excluding iron or smelting (not in game...yet?) you could have 3 types:
Small leather shields could be used for the light skirmisher style individual, still providing a small def bonus but retaining the speed and lightweight abilities of leather.
Bone could be the medium style blocking tool, with average defense and weight. This could also be good against slicing/piercing type weapons.
Wood could be the heaviest style, best defense, but make you slower. Could be better against, say, crushing but not as good as chopping?
Adding types of shields would add depth to character variations and logistics. They would be a must if ranged combat (bows to throwing) were to be added. They would be essential for balance. I imagine small bands of players having variable damage types, putting the shield bearers up front. Also imagine phalanx type combat, where shield bearers are defensive characters protecting, say, ranged people.
They should not be an "insta win" item...one should have to have the proper skill points AND physically hit the button to use it when intended...rather than just simply equip it and get all of the perks.