The fan developers behind the PC port of The Legend of Zelda: Ocarina of Time, Ship of Harkinian, have already added tons of impressive new features to the classic game and they're continuing to add a whole lot more, including a pet dog, a nightmarish multiplayer mode, and support for better graphics.
I'll emphasize support for better graphics, because the latest update simply allows you to import custom models and high resolution textures into the game. It'll be up to modders to actually create the content to make the game look better, but some work-in-progress models are already looking impressive. You can see a substantial demonstration at the 12:15 mark in the video below.
In 'I'm not sure I actually want this' updates, there's also a new multiplayer mode, where a second player can take control of a fairy companion named Ivan. As Ivan, you have access to Link's items, which means you can do things like trigger switches or help defeat enemies. It also means you can just throw bombs over and over at player 1's Link. This surely will not ruin any friendships.
But perhaps the most important thing is this: Link can have a pet dog now. Yes, you can get one of those little Scottish Terrier-looking buddies from Castle Town to follow you all around Hyrule. Now I'm desperately hoping that Zelda Tears of the Kingdom offers a similar feature.
Ship of Harkinian has already added tons of quality-of-life improvements to Ocarina of Time - like the ability to shut Navi up - and like the Mario 64 PC port has so far avoided running afoul of Nintendo's famously trigger-happy lawyers. You have to provide your own ROM for the port to rip the assets from, so the devs are making strict efforts to avoid distributing anything directly owned by Nintendo.
After fans recently finished decompiling the source code, Paper Mario could be the next N64 classic to get a PC port.