A few random ideas. Admittedly i don't have much experience with the items in the first suggestion, so other people might be more suitable to come up with better ideas for them.
Bone Helm, Bone Glove
I feel they're slightly lacking compared to the rest of the pre-HM Expert Mode accessories. The issue might be that they get less useful in Master mode compared to the rest of the accessories, since everything gets more HP but these deal the same damage.
Increase their damage by 5 in Master Mode only
Increase the stacks of common materials/consumables from 99 to 999
There are many materials out there that i often get multiple stacks of, when there isn't really a point in them stacking to just 99. They just end up eating extra inventory and chest slots.
These are the most common offenders: Mushrooms, Stingers, Jungle Spores, Vertabrae, Rotten Chunk, Pixie Dust, Ichor/Cursed Flames, Spider Fang, Ectoplasm, Beetle Husk, All Bombs (including Wet Bombs, Scarab Bombs, Grenades and Dynamite), Solar Tablet Fragments.
Traveling Merchant reminder
Don't know how necessary this change is. Often times when the Travelling Merchant arrives i'm in the middle of spelunking, farming or doing something else. Only for the end of the day to come and see him depart, when i would've talked with him if i hadn't forgotten.
Add a reminder in the middle of the day that the Traveling Merchant is doing business with X NPC that he's nearby with, or that he's waiting for you or something.
Idea List Version 2: Featuring New items!*
I agree that quite a few of these can be buffed. There are some things I don't really agree with, like giving so many weapons autoswing, some changes aren't really needed or barely improve the damage, and some weapons need more than just a stat change. But yes, most of these need some kind of improvement.
The Queen Spider Staff is really strong. I honestly think people just underuse this weapon, they see the base damage on it and don't even try it. J Bame mentioned this already, you get it crazy early in hardmode, and it hits like a truck - you get damage from the intial egg projectile, the multiple baby spiders tha spawn from it and to top it all off it deals the venom debuff (when you usually get access to venom post-plantera). There are the issues you mentioned, the short range and the gimmicky nature of sentries. The short range isn't that much of a problem, neither are the missed egg shots, since the baby spiders deal most of the damage, and they spread out and home in on enemies - in this sense it has the best accuracy out of all shooting sentries. On the other hand I agree that an instant attack on summon will be very useful.
Houndius Shootius
An instant attack will be great here too. Besides that, I think the issue with this weapon isn't the damage, but the accuracy. It's common for this weapon to miss against bats, demons, hornets, and even slime when they're jumping. I think it's best for it to home in on enemies very slightly, nothing crazy like the chlorophyte bullets, just let it turn slightly so it can hit flying enemies more often.
I think the same can be said about the staff of the Staff of the Frost Hydra, but i haven't used it much lately, but if someone who has used it recently reads this, can you reply with your opinion on it?
Sniper Rifle
To be honest i don't really know what's the use of the Sniper outside of using it for fun and in PVP, it's not really practical - it's not exactly good to fight bosses with, and it isn't made for crowds of enemies. It's good for killing singular enemies i guess, but at that point of progression that's not an issue i think, unless you're fighting problematic enemies that need to be dealt with quickly, like the Alien Queen, Flow Invaders, Casters in the HM Dungeon etc. I think if the Sniper specialzies in one shoting these guys it would give it a specific use.
This idea isn't in the game to my knowledge and i do not know how hard it is to code in, but what if critical hits from the sniper dealt 2.5 times the damage instead of 2 times the damage? This way it can one shot even more enemies, i think.
Autofire on this would actually make it useful against bosses, giving it another use outside of singular enemies and PVP.
Speaking about autofire on ranged weapons, I might be in the minority here, but i actually find it fun to spam click on handgun type weapons. But I do see how it can get annoying.
Eitherway, another idea i'll throw in is making the Rifle Scope give ranged weapons autofire. Admittedly it's post-Plantera, the only weapons to benefit from this change is the Sniper Rifle, Tactical Shotgun and Venus Magnum, but i don't think it will make any of them busted, it just buffs their general viablility.
Gravity Globe
You get inifinite flight from the Soaring Insignia already, and this item fulfills a similar function although slightly worse. As it's now it's like another "Trophy" item for beating the game, and besides, you already get the Celestial Starboard from Moon Lord ever since 1.4.
What if we move this to be a Lunatic Cultist Expert mode drop? The Cultist will finally drop something in expert mode, and it offers an alternative to the Soaring Insignia, although quite a bit different. I do not know how many people will actually go for it over the Insignia outside of speedrunners maybe, but it offers more build variety which i think is good.