In future updates, I would like to use upviral to post offers/giveaways on the sites of others, especially authority sites? Ex: If there was an article on my topic published by a famous website, I could use upviral to get their link, and when I shared the link to the article with others (on Facebook, Twitter, Emails etc), have my giveaway/offer pop up over this article? (Hopefully this made since) I saw this feature on a software name Engagifire.
I think that's exactly why we would have the integration part - so you can use 3rd party tools in combination of UpViral.
I think adding a feature like this would be too specific (wouldn't apply to most users), and therefore wouldn't make much sense. However, you could use another tool / plugin that already allows to put opt-in forms over other people's websites (there are quite a few of those, if I'm not mistaken) and integrate them with UpViral. Hope that helps :)