I want to make sure this issue is up to date with the previous discussions on this topic I have found. First is issue #5269 which discusses multiple icon improvements (some of which have been implemented in 1.0) including potential replacements for the command palette. I also found this comment on PR #5920 helpful for understanding why previous attempts have failed and what the conceptual goals for the command palette are.
After reviewing existing Material Icons, I've found that they do not have an icon that closely communicates what the command palette does. Here are two options based on existing icons and the idea that the a good icon for the command palette represents it as a "set of things you can use in your work."
The first icon combines the searching aspect of the command palette with the standard icon for a command. Its main weakness is that it focuses on code when the command palette has a greater scope than code alone.
The second icon continues to reference searching and replaces the reference to code with Material Icons' existing icon "list." This approach more closely addresses the "set of things" idea. Its main weakness is that representing the command palette as a list may be too vague.