A graphical user interface for the commands.
Yes, Horizon is pretty nice, but there are lots of things that it doesn't allow you to do. Additionally, the CLI commands are pretty intimidating. Here's the documentation for the current release: http://docs.openstack.org/cli-reference/content/openstackclient_commands.html . Notice how each command has at least 20 options, each with its own flags, etc. And Horizon doesn't actually call these commands. It uses the REST API (and/or other API), which is completely separate.
So what I'd like to see is something graphical that lets you first pick a command ('nova', 'glance', etc.), then you have a drop-down choice list for the next parameter, and the next, etc. Many of these commands require output from other commands as their input. So the idea would be that the interface is running each of the required commands to build the appropriate list of options. And after building the command, this tool could output that for you to use in scripts, etc.
If it was an easy task, it would already exist, and it doesn't. There's also quite a lot of movement in the code, so keeping this tool up-to-date would be quite a challenge. But I personally think it would be worth it.