Well I am not sure this is classified as ‘outside the box’, but one of the most common link building methods currently employed by some of the biggest bingo brands in the UK, is to publish editorials on established and high authoritative newspaper websites. In addition to benefiting your SERP, the brands get direct traffic, as this newspaper websites have a lot of traffic. The only drawback is they cost a small fortune. If anyone has tried this method please feel free to chip in.
Moreover, a lot has been made of adding more digital SEO to your efforts, like producing videos. Not sure how that helps but it seems to be gaining ground.