Almost all value fields already do this, but allowing the use of variables for every value field on every action/condition/trigger increases the versatility of GameCreator exponentially for every field changed in this manner.
As mentioned, you already seem to do this pretty much all the time, but just making it a rule that is always followed on future actions/conditions/triggers would add a nice consistency.
Thus far, every time I run into a problem that requires a custom action, it has been because of this issue.
Examples of Actions that only allow static fields:
All Vector3 related Actions - none of them seem to let you set individual values of XYZ from variables
Random Value - min/max values are static fields
Wait - wait time is a static field
Examples of Triggers that only allow static fields:
On Player Stay Timeout - (duration is a static field)
All On Hold related Triggers - (hold durations are a static field)
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