Something I just thought of. What if there is a kit called /kit random and can only be used every 2 days. This would give the player a random item. ranging from cobblestone or even a diamond (gets more rare by value). I know the voter crate is similar, but it's cobblestone 90% of the time and some redstone or glass. This also allows non-donors to receive iron or diamonds without trading ridiculous amounts of items. If you want to keep items rare, make them rare to get.
No support from me, just not a good idea. Just vote and hope you get lucky, I've seen my friends get a spawner out of voting. Besides, you can trade for rare items if you build up grass by voting, selling items, etc.
This is just like voting crate, I think this is not necessary as it's not that hard to get 25 grass for a diamond. Just keep voting, theres a 1/500 chance on getting a creeper head I think.
No support. This may contribute to inflation, anyways. Random items can be generated from the voter crates or voting.