One thing I love about FPS/TPS's is the sound of a bullet. Purely cosmetic, of course, but I would love to duck then hear a bunch of metallic twangs from above.
Secondly, different block types. The basic block type would cost 1 block from your inventory, abosrb three shots, and make an impact sound like dirt.
The next block type costs 2 blocks, absorbs 5 shots, and makes an impact sound like chain link. It also has a latticework texture and can be shot through on one side.
The last block type costs 3 blocks, absorbs 7 shots, and makes an impact noise like steel or sheet metal.
If the idea floats I'll make block mockups. I would've made seperate suggestions for the sound effect + block types, but they go hand in hand, I think.