Now that I've had more time to play around, I have some suggestions for items.
First up, the
Sergeant United Shield. As far as normal attacks go I think it's good, though it can sometimes spend way too long in the air if enemies are scattered. The main problem is the right click parry which, as with the
Brand of the Inferno, is far too tight on timing to be useful. I propose making the timing more generous, and/or removing the movement speed penalty, which makes it very hard to use. If necessary, reduce the payoff or increase the cooldown; a weak effect that can be gained reliably is better than a strong effect which is nigh impossible to get. For that matter, the Brand of the Inferno would benefit from this change as well, and almost certainly wouldn't need any nerfs to counterbalance it).
I think the
Light Discs could lose the stacking mechanic they share with the Bananarangs (adjusting the crafting cost accordingly). It makes some sense for the Bananarangs because Bananarangs are hard to farm, but nobody ever uses a partial stack of Light Disks because the Destroyer drops enough materials for a full set. Making them a single item would let them be reforged, a nice boost to their power at a time when the melee class has limited options for handling the increasingly common enemies and bosses that keep their distance. Given that being able to craft Light Discs also means being able to craft the Megashark, I don't think they'll suddenly become too strong.
The
Dart Pistol,
Dart Rifle,
Crystal Darts,
Cursed Darts, and
Ichor Darts I have mentioned previously, but I'm making a more formal argument now. Prior to 1.4 they were already very strong items thanks to the properties of the ammo. The near-homing ricochets of Crystal Darts made them great for invasions and general exploration (the tier 2 Old One's Army could virtually be soloed by standing in one spot and firing Crystal Darts in one direction if there was a vertical wall to bounce them back), and were also good against Skeleton Prime since he has so many parts. Cursed Darts are similar to the North Pole, carpeting the ground in fire to kill invasions and decimating the Destroyer. Ichor Darts inflict the valuable Ichor debuff and their displaced shotgun spread let them do high damage to a single target from a distance. You could use darts reliably through the mech bosses and Plantera, maybe on the Golem too. They're also extremely accessible, requiring only the ability to kill Corrupt/Crimson Mimics and crafted by hand in large quantities from common materials. Why they got a buff is beyond me. What they lack is endgame viability; by the time you get past Golem their special properties are no longer able to make up for their relatively low damage, even after 1.4's buff, resulting in the loss of an interesting and fun weapon type. If it were up to me, I would revert the buffs and add a new stronger dart gun somewhere post Golem. Nothing fancy, just a Dart Rifle/Dart Pistol with the numbers buffed.
The
Razorblade Typhoon is frankly too good. The main issue is the extreme versatility; no matter what I'm doing, I've never found the Razorblade Typhoon to be anything less than a good option, and usually it's amazing. And that was back in the 1.2.4 days, since in 1.3 its mana cost was effectively halved for some unknowable reason. I think that buff should be reverted.
The
Staff of the Frost Hydra currently is the weakest of the biome chest items. The main problem it faces is that, at the time you get it, your main challenges are either the enemy hordes of invasions or bosses that zip around the screen at high speeds. A sentry that damages one target at a time and has relatively low accuracy has little value, especially when the Tavernkeep provides sentries of comparable power that are much easier to acquire. I believe accuracy is the issue that's easiest to address, either increasing the projectile's velocity or giving it some kind of homing ability.
My most ambitious suggestion is about
Cactus Armor. Currently the set is stuck in a limbo where it's slightly better than Wood armor but not as good as Copper armor or better, meaning it has no real purpose other than slightly rewarding people who go to the slight effort of gathering cactus. Why not make it the early Summoner armor we all want? Replace the current set bonus with +1 minion or even just +x% summon damage, and the Summoner class is much more viable from the start of the game. The paltry 3 Defense it would have ensures that other classes won't be as keen on stealing it, because at the time every point of Defense you can scrape together matters.
I'll probably have more suggestions in the future, there's still plenty left to experiment with.