One could close off the postgame dungeons by removing the exits to them in the map feature data. Not sure how easy it would be to keep them locked until all four Crsytals are lit, but that would be another possibility.
There's a lot of things you'd need to do to bring FF1a closer to Origins. Off the top of my head there's spell formulas, the XP table, changing/removing new items like Ethers and [Stat] Plus, and making monsters use spells more often (Monsters in FF1a roll out of 200 instead of 128 when determining whether to cast a spell); those are matters for a separate patch. Perhaps somebody else will make a project in that direction - my own is going to stick with the MP system.