Certain that implementing this feature would be complicated as hell, I'm nevertheless going to throw it out there, because I know I'm not the only one to have asked (and Marketo isn't the only tool that doesn't support this): one of the major limitations in the Engagement Nurture is the lack of ability to set a resend policy. Such a policy requires separate, subordinate campaigns, which does not scale well at all, and the logic is nearly as convoluted as the original Traffic Cop concept (before the Engagement Engine was available). However, when we create nurture programs, a resend policy is a de facto part of the structure, so this has been painful for us.
The logic is pretty self explanatory:
If was sent email x
Wait n days/weeks
If not opened email x, send email x' (new subject line)
Of course, with potentially dozens of pieces of content in any given nurture stream, I imagine this could get complex really quickly, but I envision something like this:
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