That would just give your shield the inherit properties of those metals like their sturdiness, or ability to deflect certain things better. If you want a shield with spikes you would need to buy a shield with long spikes in them those exist. Absorbing a kite shield would make it into the shape of a kite shield, same with a long, or buckler shield. Naofumi's ability to absorb raw materials gave him the ability to use those and gain his shield those attributes and absorbing generic shields just made his shield stronger. Absorbing monsters is where his truly useful abilities came from. Other than that there is clear RPG Qualities where generic standalone abilities are there like Meteor Strike, or piercing arrows from the bow. You would I imagine have the ability to debuff, or buff your own defenses.