When I was a teenager (from about the ages of 12 to 19), I was way more into anime than I am now. It was so intense that it was pretty much to the exclusion of every other form of entertainment. It's a shame that anime streaming sites weren't around back then, because being able to watch my favorite anime anytime without having to buy DVDs or catch it on Toonami would have been the best thing since sliced bread at that point in my life.
I could easily picture a younger me becoming completely obsessed with Tiger & Bunny and Puella Magi Madoka Magica, which were my only favorite anime in recent years. The fact that they didn't develop into special interests despite how much I enjoyed them was one of the first things that tipped me off to the fact that maybe, just maybe, I have been outgrowing my love for anime. I mean yeah, I like to go around on WP and declare myself to be an anime fan, but the truth is that I rarely ever watch it anymore and instead of saying "Oh, this series looks good", I instead say, "I wish this had been around when I was younger. I would have gone nuts for it then!"
Despite my exclusive love for anime back then, I could also picture Adventure Time becoming a special interest for my younger self. It has a little bit of everything my younger self went crazy for. I was actually pretty disappointed that it didn't develop into a special interest at my current stage of life, though once again, it's a sign that my tastes are gradually changing.
TL;DR - What are some things that you find pretty cool now, but wish had been around when you were younger because you just know it would have turned into a special interest?