I think that it would be nice that you can exchange your iron, gems and mithril into gold. Because there is no event going on for quite a big time and you're only saving all of it and it isn't really relevant at that time.
Well the other materials are eventually used for higher upgrades but I still think this is a good idea that will allow newer users to get upgrades just a bit faster.
Regardless it's good to see you're still planning on developing your game, I was kinda worried the website would shut down by the time I'll be done with all the upgrades.