Question: I would love to have the following setup: a chromecast-style dongle running some emulator with a bunch of roms, plugged into the HDMI port of a tv; a couple of wifi controllers to play.
Is it possible?
It's called Raspberry Pi.
Check out PiMAME or RetroPie. The Raspberry Pi can emulate a lot of the stuff out there.
Most of the things it can't emulate are too recent anyway.