There's already a lot of suggestions of people trying to make more use of labels. On Gmail, I was able to associate emails with a project (following the GTD method) by applying a label to them, but this also requires fast access to the label, ideally from the search bar.

To reproduce:
1. Have a label called "project-123"
2. Have an email with the title "Important Information regarding Project 123" in your archive with this label
3. Type "project-123" into the web/mobile search bar and search

Expected: "Important Information regarding Project 123" shows up in the search results
Actual: Empty results

Note: Web has a workaround by being able to select a tag from the dropdown (which means the functionality of the search finding these emails exists) but it's hard to accept as a complication and is not an option on mobile at all.