I didn't see a conversation in this topic and, as an avid fan of these kinds of systems, I feel like poking TheWizard to see what his current ideas are at for it and where they could develop as we use this thread to openly discuss these elements.
I'm especially an avid fan of the 3 planes forging system. The first plane of crafting being 'item 1 + item 2 = item 3. Second plane of it being item 1 + item 1 = Item 1 at higher level/new item. and third plane being the enchantment plane of Item 1 + Item 2 = Item 2 with new traits. And that's the basic details of it, it can get much more thorough when adding in multiple item recipes and recipe chains.
Item chains like these are the reasons why some grinders like Runescape are notoriously popular. What detail and level is up for toning I'd guess though.