I'd suggest keeping it to the current monthly system but don't have items that you can buy in the shop as a prize. Keep it to stuff inaccessible from the shop, like the stuff you guys have been doing thus far with armor and weapons in August and Enchanting books this month. If you use items that can be bought from the shop already people may tend to think "No real point in voting, I can already buy that from the shop whenever I need to."
A few suggestions:
Horse stuff (probably put some nonhorse stuff with it so people that don't dig horses too much still have motivation to vote)
Enchanting Books
Armor/Swords (Consider using chainmail, which is fairly rare)
Silk Touch Shears (Some people really want cobwebs and ST is fairly tough to get on a book)
XP bottles (Probably as a side to something else)
Minerals in ore form (Gives the winner choice in whether to convert it or place it as an actual ore block, which is different from the purchasable minerals in the shop)
