Well see, with the slingshot, you can't aim. It's mainly just for fast shooting. With the bow, I've made a new aiming system that's new to 2D Zelda games AFAIK, not to mention it's more powerful, and shoots faster. You can aim in the four directions, along with any angle in-between, allowing for more precise aiming. The hookshot in my game features the same aiming system as well.
With most of my items, they're either already in previous games, or they're "reworks" of old ones. When I first started development, I had a list of completely new items, but most of them only had use in the dungeon they were found in. Not to mention, they were based off of the element of said dungeon. For instance, I had an item called the "Water Whip" which was found in the Sunken Grotto (Water Temple in my game), that could drain pools of water, and then fill another. It just didn't have much of a use outside of the temple...
So I revamped the list, adding in items from previous games, like the Slingshot, Deku Nuts, etc. (They were never in it originally). Then I added a few new items in, along with some "fun" items, like the Fishing Rod and Pictobox.
I could always have weapon upgrades for some things, like maybe the Slingshot could be upgraded to fire 3 Deku Seeds at once, instead of 3...Or the Hookshot could be upgraded to shoot farther, like the Longshot. IDK, I'm usually against weapon upgrades, as they can end up being overpowered, but then again, that's up to me.