Once you pass a certain point, you end up accumulating way more imagination than you can spend. You've extended and cultivated your yard and street as far as you can, your upgrades hand contains nothing but keepables and energy tank upgrades, and the only purpose img continues to serve is to elevate you to higher and higher levels. Which is no bad thing, necessarily, but wouldn't it be nice to be able to spend that img on something as well? Here's a couple of ideas:
1. Convert img into skill learning time. While you're learning a skill, you can spend a big stack of img to speed up the process. You're basically imagining yourself into a more skilled Glitch - creative visualisation taken literally!
2. Use img to restore cultivated items or expand your house/tower. Again, you imagine the outcome hard enough and it happens! Instead of contributing one or more of the components, or performing the units of work with the requested tool, you dump in a big pot of imagination instead.
The feature could be accessed via learning a particular skill and/or buying a really expensive upgrade: if you have 250k img to blow on this particular upgrade, you're probably not waiting to spend it on anything else. Maybe it could also unlock at a particular level. Glitchen who can't/don't want to spend img on the activities above don't have to because they will still be able to access these aspects of the game in the way they do now.
I think this is totally consistent with the idea of Ur as a place that we as players create from our own imaginations. Details of conversion factors and game balance left as an exercise for the game designers :)