There are many more than 6, but I like the Duolingo model. I also like Badges.
I like the idea of applying a 'simple' game goal to learning and watching what happens. Tower defense is about keeping the baddies from getting across your territory --- you build 'towers', upgrade them, spend resources on heroes, traps, and other stuff to help you defend your space.
Simple. Don't let the baddies get through.
But when you work out all the layers and dynamics, any tower defense game is rich and multidimensional (I say there are about 11 dimensions in TD games). How can this tool, this game type, be applied to learning (online and brick/mortar)? There's some movement going on there already. But how about a tower defense version of cellular functions? Upgrade your mitochondria so you have enough power to divide before the viruses damage you beyond repair, etc. And tower defense for orbital mechanics -- your space stations defend vehicles and planets from space debris and maverick rocks. Great content can be folded into this genre.
Thinking on it. Here are some things I like: