So let's say that you're playing on a theoretical ultimate pack that has all the mods in both the direwolf20 pack and the mindcrack pack. Let's say that you've decided to embark into a void world and throw away your return linking book, but you'll allow yourself to bring in any items from the overworld to create a giant factory. Given enough time and with the proper setup, what can't you produce an infinite amount of?
I've been looking through NEI and several wikis for a while now and I can only find very few "useful" items that you can't produce: Oil, Uranium, and Mystcraft Crystal Blocks. Thaumic Shards appear impossible to produce, but they can be acquired by a rare trade with a Thaumcraft villager, or the Thaumonomicon states that "It has also been known to happen for normal stone to spontaneously turn into infused ore, but only if the local aura reaches extremely high levels".
There's unfortunately a number of decorative blocks that are just flat out, like End Stone and every XyCraft block.
I've been looking through NEI and several wikis for a while now and I can only find very few "useful" items that you can't produce: Oil, Uranium, and Mystcraft Crystal Blocks. Thaumic Shards appear impossible to produce, but they can be acquired by a rare trade with a Thaumcraft villager, or the Thaumonomicon states that "It has also been known to happen for normal stone to spontaneously turn into infused ore, but only if the local aura reaches extremely high levels".
There's unfortunately a number of decorative blocks that are just flat out, like End Stone and every XyCraft block.