dnd 5e – D&D 5e inspiration dice as currency for PCs to improve their characters?

Has anyone heard of, or thought out, or even used D&D 5e inspiration dice as currency for PCs to improve their characters? I allow PCs to accumulate them, so it occurred to me, “why not let them buy a skill, feat, spell slot, etc… for ‘x’ number of I-Dice? Thoughts? Possible costs in I-Dice for each?”