There already is one. Law 15.17 when the ball becomes playable and the ref calls use it you have 5 secs, otherwise a scrum is awarded. Just no ref will ever blow up for it. Just like no ref will blow for players going off their feet at the ruck. Something which did actually happen on Saturday much to my surprise!
Edit: here is the actual wording of the law
'When the ball has been clearly won by a team at the ruck, and is available to be played, the referee calls “use it”, after which the ball must be played away from the ruck within five seconds. Sanction: Scrum.'
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