there is a lot of ingenious engines. The question is which one can be made into ...

there is a lot of ingenious engines. The question is which one can be made into practically usable product.

There is even productized version of rotary-vane engine coming supposedly next year in a Russian car:

I'd be though very surprised if they pull it off as beside Mazda rotary i don't know other mass-produced cars with non-traditional engines.