My idea would be the addition of titanium and steel,crafting reciepe as follows:
Steel:
Iron Iron Iron
Iron Carbon Iron
Iron Iron Iron

All iron above being ingots.

Carbon can be obtained by cooking coal with ONLY LAVA as the fuel.

Titanium:
Mined from the ground, rarer than iron, slightly more common than diamonds.

Steel can be crafted into weaponry and armour.

Titanium ore can be smelted into titanium ingots.

Titanium ingots can be crafted with steel ingots to make heavy duty metal:

Titanium Steel Titanium
Steel Titanium Steel
Titanium Steel Titanium

Heavy duty metal can be crafted into tools and armour.

Heavy Duty Armour and Tools = 25% more durability than diamond.

Heavy Duty Armour and Tools = 15% less effective than diamond.

That's all for now, if I think of more I'll add it.