Some items that could be placed:

Dirt, Sand, Bricks, Emerald, Mob Heads (human heads + Player Head Plugin + /hat = Epic) , Obsidian, Nether Stars, etc...

Things Like That Would Improve The SkyBlock Gameplay, Thanks for reading

Sorry about my english I am using a Translator

 

first off sand= never dirt=never bricks you can u buy clay mob heads nope and emerald spawn villagers on .org .net you cant atm

 

Also obsidian you can buy from players. It is a legit trade