Q:

Could you create a game for an older system (say, SNES) and sell it with an emulator as well as the game?

I was looking into development for the SNES in assembly, as programming in assembly is my hobby. I then had the thought: what if I could sell my native SNES games for modern use (on steam, bundled with an emulator for example)?

A:

Yes there should be nothing illegal about emulation in itself (any virtual machines such as Java VM would similarly be illegal, if unsure ask a lawyer) and in this instance the SNES has no BIOS so that isn't an issue either but you'll need to license the rights to the emulator (if it does not permit commercial use freely already) or write the emulator yourself.
And you'll likely have to add some custom features to make the emulator and emulated game more user-friendly for the computer user experience, for example:

Auto-boot your game with the emulator locked to your game. 
Allow the game to give achievements?
Cloud saves?

It certainly has been done before with other platforms. For example Castlevania: Symphony of the Night for XBox 360, and classic games released on Steam already use bundled emulators such as those by Sega.

