There are many emulators for old computer and game console systems (like C64, ZX Spectrum, Amstrad CPC, NES, Sega, etc). But all I know about are free and open-source software. I was not able to find any commercial ones. Are there any legal issues for creating commercial emulation software for old systems?
Almost any IP issue I can think off (trademarks, copyright, patents and trade secrets) would apply to free versions as well as commercial versions.
It might be the case that a for-profit operation would attract greater enforcement but the legal technicalities would be about the same with a free version.