Those are pretty good suggestions, but a few take a way from the game play of survival, like Anxiety said. Such as lava and water buckets, in which are not hard to get once you have dug deep enough or made a nether portal. Vines, in which you can find a jungle and use some shears on them. And books, in which you have to nab a few cow and harvest some cane. One thing though, they do not allow book and quill to be made or sold because of people "book banning". Meaning that they put in some funky command, and ban someone from the server. But some of these may come in handy in Sky Block to cut some crafting corners and also because some of these items are hard to obtain. I'm not sure which ones Sky Block doesn't have, but I'm sure they are all covered.
I hope this helped,
Sam (samara1978)