Plugin that will popup subscribe form and will generate unique coupon... Theres somethin like that ?

I do some similar things with coupons.

The easiest is not having a unique one and just taking someone to a thank you page that has a coupon on it that looks unique.

Another way is using some coding usually in php that generates specific fields that are dependant of passing parameters from your forms to the thank you page.

Another way is creating a coupon inside your auto-responder and inserting field names into the coupon that is emailed to the respondent. The thank you page says check your email for the coupon.

Yet another way is using PURLs or personalized URLs. Some systems like Ontraport formerly Office Auto Pilot have this feature.

I use a system that Mike Capuzzi has that is wordpress based but you would have to host the site (i.e. the thank you page where you had the coupon) on his system. There is an example of one I use below.
You can generally use any name or field PLUS a modulator Plus the domain name. So for the purposes of demonstration if you typed any name it will work so you can send the parameter from your email auto responder.

My example for you but you can change the name or the modulator if you like is

http://guyseo.pictureframing.superbonusfrom.me

You could have a coupon with specific fields and they could be personalised on the fly if that suited your purpose. They could then print the coupon or click to buy and enter a coupon code you set up.

Some food for thought.