I've been playing Minecraft longer that I can accurately remember, and singleplayer is just getting...boring. There are too many things to craft, and possibly not enough bosses. I want something a bit more satisfying than defeating one dragon and one three headed zombie snake thing. I've got Forge, but honestly even creating Nether Castles isn't fun or a challenge. I'm no redstone master, and have no plans for it. I've tried Adventure maps but they don't really fit my play style. The thing is, Minecraft has so much to offer, and 99 percent of it I've done. When does Story Mode get released? Oh, and if you find my signature slightly ironic like I did I've already done all of my own challenges.
The Meaning of Life, the Universe, and Everything.
Play some hardcore modpacks or make your own. Heck, make it so you post progress on it, and people can post challenges to you in it. There are loads of mods that add bosses, alternate creatures, better/harder mobs and AI. If you've cleared as much vanilla content as you want to, then you need to change the content.