Sounds great, thank you.
After thought: Do you mean that players will have to wait until they get to the finished product before they can get a modified item and intermediate items will have no affect on that?
I think this is how Wurm Online got around the stacking problem: each time a special version, a rare, of an item is made, it is given a set modifier which allows rare items to stack. However, I like the PS idea of a variable modifiers better though I see how it makes a problem with intermediary items.
The way Wurm seems to have dealt with intermediary items is that the item is rare but has no end state modifers in and of itself. What it does have is a fixed chance to pass on "rare" status to the next item up the ladder. If I understand it correctly, this makes it so that all "rare" intermediates of the same item are actually the same kind of "rare", allowing for stacking of items. This also means that the end result item has a better chance of becoming rare if more rare intermediates are used.
Part of the reason I bother to bring this up is that rewards schedules can help players get through tedious/repetitive mechanics. However, for it to work it needs to happen on a regular schedule even if the precise timing unpredictable. For instance, if it takes days and day to get special items then it will lose the power of anticipation to help ease the tedium, but if someone knows they will get at least one a day it is a better incentive. I just randomly choose those time frames to illustrate a schedule.
If it is possible to not flood inventories with unstackables this way, could it be done so that the intermediates do have a special status and give a fixed chance to make the next item made from them special? The intent is to give players periodic "rewards" throughout the whole crafting process.