Fpgas are very unsuitable general purpose computers. De10 nano is a very beefy fpga and can barely play doom acting like a general purpose 486 and lacks a fpga
What I would find fascinating is to think about languages and OSs for FPGA and try to break loose from the utterly boring C/Unix/68k,x86,MIPS,blah paradigm. Having something turnkey with attachment to various ports and a display would be kind of cool. Maybe that exists already.
I worked on a project with a processor-on-an-FPGA and while it wasn't fast, the video codec in FPGA made it useful. Kind of a neat thing I think.