You can already do this a multitude of ways in Rule Machine. Here is one:
Trigger event: Motion active
Actions to run:
Wait for event: Motion inactive
Wait for event: elapsed time --> 0:05:00
Log: "Motion has stayed inactive for 5 minutes. Do something now."
This may look confusing because it triggers on "motion active," but keep in mind that the primary purpose of a trigger is to make your actions run, and you can't get a "motion inactive" event without a "motion active" event first (though this does mean you'll need to wait for one of these the first time after you install this rule). Also, keep in mind that "Waits" are cancelled any time a rule re-triggers, which is why I've chosen that trigger here and used "Wait" over "Delay" for the timer--easier to not need to cancel anything.
I do see the possible appeal of somehow combining those first two actions into one (sort of a "wait for event plus elapsed time" thing, where the while thing would be cancelled as normal if both don't happen before re-triggering), but I could see that being confusing from both a UI and end user perspective, and it's only a few more clicks to get that other action with the same end result. Either way, not my call. 
