Will you be ready for it?
On-site decisions can be penalized without worrying about "negative SEO," for on-site decisions are those of the webmaster.
Can Google implement this?
Yes, it would be a walk in the park for Google's engineers. Detecting opt-in boxes is easy.
Would Google want to implement this?
I guess it depends on how cynical you are.
99% of opt-in boxes have a single purpose: monetization. "The money is in the list," etc. Google wants a cut of your monetization efforts, and opt-in boxes get around that.
99% of opt-in boxes have a single class of user: Internet marketers. Google wants you to be attached to the hip of Google in your online-marketing efforts, and opt-in boxes get around that.
Consider this: Google already frowns on its very own AdWords advertisers who engage in "information harvesting." Depending on which section of Google's policies you read, that may or may not be qualified with "primary purpose of the site." Here's a prominent section where it's not qualified:
|Free Gifts: The type of websites that are not allowed includes, but is not limited to, websites offering free gifts or other incentives in exchange for personal information. We refer to this practice as "information harvesting". Examples of free gifts or incentives include prizes, offers, videos, eBooks and quiz/survey results.|
Anyway, the important thing to note is that non-paying marketers who depend on SEO efforts can't really expect a better deal than paying marketers who are throwing money at Google.
I guess that leaves one question:
What incentive would Google have not to penalize little-guy sites with opt-in boxes?
I await your thoughts.