Featherfall is a pretty handy potion, it really is under rated. It's what let me complete a pre-hardmode item only run pre-1.3 (which was basically phoenix blaster most of hardmode). Wings are not vital, but they make life a lot easier.
I'm neutral on this idea, though. Once you get wings, you can easily go without boots, although it is bothersome to walk so slowly in your house. Since you can already get the horizontal boost of boots from (most) wings, I wouldn't call it overpowered to allow hermes sprinting with wings. It would just make life at home more convenient.
To those claiming this would be OP.. I'm not so sure on that. I've played this game since days after release, and there was once a time that modifiers didn't exist, so no +20% damage or +20 defense, or extra crit (which crits also didn't exist) and speed on weapons. We also didn't have accessory tinkers, and we were once without buff potions. My preferred "load out" was Hermes and Rocket Boots, Band of Regen/Flippers, and CiaB with the balloon. Now, I'd go with Yellow/White Horseshoe Balloon, Flying Carpet, Frostspark Boots, Band of Regen, and then just whatever else that could be useful.. now with just 3 accessories I'm more powerful than I was pre-1.1, and that's not factoring modifiers, or the fact that they've nerfed lots of enemies and some bosses since, or that we have campfires and heart lanterns now, or honey, or that we have buff potions, or that we have mounts now. If the game is still fine now, would allowing a wing slot suddenly break the game? I'm leaning towards No. Now that may seem like a slippery slope, because then you could ask if allowing a slot for X item would break the game.. but seeing how bosses seem to be designed with the mobility of wings in mind, they're the most worthy of their own slot to me. You can fight them without wings, but you're really just making things harder for yourself. Going without wings for something else is less of "another option," but more of a handicap.
About shields.. I dunno why so many seem to hate knockback so much. Besides being knocked off a high ledge to my death by a bat in the early early game, knockback has saved me more than harmed me (there was one time a bat bumped me out of a falling boulder
). It bumps you back to avoid further damage. If an enemy is on you, you will continue taking damage, but the knockback gives you a chance to escape. Spaz's fire breath is the only attack that I think no knockback would be beneficial, though it's already so easy to avoid so I don't even bother. I think the only time I've ever wanted to use a cobalt shield, besides its awesome vanity purposes, is for the Dungeon Guardian, where being bumped back by a slime can kill you, haha.. learned that the hard way.
I think I'd be down for a wing slot. The game does seem balanced around them later on and they're a special kind of accessory with several variants with increasing power, similar to armor. A playthrough without wings is a lot more challenging than a playthrough without an Avenger Emblem. Emblems are optional, but I wouldn't call wings vital or necessary but rather.. considerably much, much more important than emblems. At least until the Cosmic Car Key, which moves slower than wings but.. moves fast enough, really. You can out-fly the Moon Lord with it, and tapping carefully won't trigger him to teleport to you, as you remain in death ray free zone. Wings still have some purpose out of combat then, so the slot wouldn't entirely become obsolete.
TL;DR: We once had no modifiers, no tinkers, and no buffs. So I don't think a wing slot would suddenly break the game. *shrug*