There are some cases while writing your mighty expressions you suddenly realize that you forgot to add a variable (instance/local/global) to your project. Then you need to close the expression window, add a variable (in case of objects you need to find it in the project first), find an event you were working on and retype (or paste) expression. It's not hard or difficult, but it can take some time to do.
What I want to propose is to create a variables on the fly (in the editor only of course).
Objects: If you are writing an expression for specific object, then this object is already selected, like on image below.
All you need to do is, for example press right mouse button and click on "add instance variable" - then well know popup window will show up to add it. After you finish you can just simply continue writing your expression including just made variable.
Global/local variables - same as above but with one difference. Newly made variable will be place above current event - so it's easy to find and move after.
This could speed up a lot our workflows, because you would not have to remember to create all your needed variables before making your events. Just make them while you writing expressions.