I see several other suggestions hinting around the same general purpose, and if I missed this suggestion elsewhere, I apologize.

I would love to be able to have discovery items (more specifically item prototypes) in templates populate inventory fields, or even create 'on-demand' fields, so to speak. The most obvious situation I've run into that this would be an ideal solution for would be stacked switches monitored by SNMP. I currently use discovery items to pull serial numbers, ROM versions, etc. from every switch unit in the stack. If we could populate an inventory field named with, say the {#SNMPINDEX} macro, it could create serial number and software version inventory items for every unit of the stack without having to create items/templates specific to the number of units in each deployed stack. It could also accommodate stacks with more than 2 units (Since there is only Serial Number A and B currently, and our current switches can stack up to 12 units) without having to dump info into the larger text fields (like notes).

I am sure there would be other uses using the agents or user parameters, but that was the single most impacted situation I've been trying to work around currently.

This would be very closely linked to customize-able inventory fields (which I know I've seen other posts for).