I wouldn't change much to The Legend of Zelda, as it's actually pretty decent on its own. I would probably make some of the secrets, such as the Door Repairs and Rupee Moblins, a little more obvious. I would update the graphics to at least 16-bit quality since the game, original and ports, has only been in 8-bit, and also because we have better video game consoles than those that were available at The Legend of Zelda's time. That's all I can think of as of now.
Why would you change graphics? I don't see any reasoning behind that; would it make the game better? It's the gameplay that made that game great, and I think updating graphics just for the sake of it doesn't serve any real end. Why do we need to "upgrade" a game just because new technology is available? I wouldn't care for any additional glitz and glamour, it would just seem out of place. They made ports of the game because they didn't feel like the game warranted any changes; when people play a classic game, I'd imagine they wanted to play the game that they'd heard of so fondly by others. If it were changed, it wouldn't really be the same game again, then would it? This is why I'm generally against changing an old game substantially -- I don't understand the reasoning.
As my blatantly overt ramblings convey, I wouldn't change a thing. Sure, the graphics were dated and the game was hard, but I like it, dagnabbit. Now you kids get off my lawn.
I don't seem to recall the Magic Rod being all that useful. Perhaps the game could have used enemies or obstacles that this item could interact with. Also, arrows instead of rupees should be consumed when using the bow. The ability to travel diagonally would help against some of the very tough mobs like blue wizrobes and darknuts. And the Magic Rod + Book makes the Candle kinda obsolete. A Link to the Past fixed these issues though and is why I play that game much more.
Making the dungeons more difficult. Theres basically a strait path through all of the dungeons. Except for Death Mountain. That was hard. I'd also amp up the graphics, like what Pocket Asian said, at least to 16-bit. 64-bit would also be nice though. I'd make many common village areas where you could buy stuff and interact with a much major story line. Because I was lost when I played the game on the gamecube and there was NO set story line. My whole common village area wouldn't coincide with my timeline theory. (which is that after WW, the flood resisded slowly at a time, and at a certain time is when LoZ takes place with all the civilization living in the mountain tops where Link can't get to (and Ganon))
Ohh... There could be a lot of things that I could make better. But if I had to pick just one, it would be better cutscenes. It would be nice to see more of Link's infinite bravery and how much he cares for Hyrule (or anywhere else).
The game doesn't really need a whole lot of changes. I don't mind the graphics and the gameplay is pretty solid (even if the game itself is very challenging). I guess it would be nice if secrets had better visual clues. Nothing obvious, just something that makes a particular spot stand out a little bit. The only other thing I would change are the poor translations. Make the phrases the old man says more understandable. The game is hard enough, just give players hints that actually make sense.
But, dude, I'm 16, and I proudly own a ColecoVision! D:
Anyways, the thing I'd change about this game would be to add the ability to move diagonally. If such an ability were added, then that'd greatly ease the difficulty, because moving just up, down, left, and right makes it hard to avoid attacks, especially when in a room filled with Wizrobes. I can't tell you how many I've died because of that. A Link to the Past really makes you thankful for the addition of moving diagonally.
Wow, I always thought the magic rod was probably the most useful item in the game, it was my secondary weapon from the second I got it onward. Sometimes I would go straight to lev. 7 right after getting the magic whistle just to have it earlier.
One thing I would love to fix are the controls. The graphics and map I can live with, but the control over the character really made the challenge for this game. Yes the ability to move Link diagonally would be an incredible improvement and help out when walking into a room filled with Darknuts. Also I think the movement should be loosened a bit. Links walking always seemed to limited or hindered in some way. Link would move very slowly, almost as if something were slowing me down when I tried to move out of the way of a beam fired at me. Or moving around to hit a Darknut in the back. They could always switch directions so quickly and efficiently, while Link acted like he was walking around underwater.
Anything else I would want to change would be the quoted from the old man. Either through a new translation or even some localization so that things make some more sense.
What is he trying to tell me?! Seriously!
They make no sense whatsoever and the Nintendo translators back in the 80's had to have realized this. Why not tweak up the words some so they can actually give the player some kind of hint as to something they can actually accomplish instead of an exact translation that could never be understood. A guide was almost always required for the players to figure out what he meant. And these days guides for NES games are becoming a little too rare.