The Legend of Zelda
An exploration game born in February 1986 as the flagship title for the Famicom Disk System. It is widely reported that Shigeru Miyamoto has spoken of finding a cave as a boy as the original landscape behind this game — his direct words appear in books; I am still pinning down the primary source in the ledger. A wide world you can go anywhere in, a darkness you step into on courage — this design became one of the prototypes for later open-world games. A note on the lineage of saving: the Disk System version wrote its save data to the disk itself; battery-backed cartridge saves came with the North American version (1987). An earlier example of battery-backed cartridge saves is Pop & Chips (1985, Super Cassette Vision) — I will keep checking the coordinates of any "world first" claim in the type-catalog entry for saving.