I guess most or many of us know some games and DLC available at GOG that we haven't bought yet.

This can be for a variety of reasons, but for me personally, it is either about price or price due to owning elsewhere.
I'm not far off having 1,800 games at GOG, so it's not like I am stingy when it comes to purchasing games here.

Some folk I know, haven't bought a game at GOG, due to issues like 'no multiplayer' or missing elements or missing updates or lacking bug fixes or just not the version they want.

The following are games I want, but haven't been listed at a decent enough price for me yet. These are some of the ones I know I want, as opposed to ones I just have an interest in.
What are some of yours, and why?

18 Wheels of Steel Collection #1-#4
ATOM RPG: Post-apocalyptic indie game
Chasm: The Rift
Diablo + Hellfire
Dreamfall Chapters: The Final Cut
Ignition
Kim
Metal Gear Solid
PowerSlave Exhumed
Railway Empire - Japan
Screamer
Shadow Tactics: Blades of the Shogun - Aiko's Choice
Soldier of Fortune: Payback
Surviving Mars - Martian Express
Surviving Mars: Below and Beyond
Surviving Mars: In-Dome Buildings Pack
The Witness
War Craft I & II Bundle
XCOM 2: War of the Chosen - Tactical Legacy Pack
Zombasite
etc etc

Some of them I have been waiting for years to drop enough in price, but always have the same discount on a regular basis every few sales.

And of course, there is a bunch of more recent games, still in a state of newness that will take a while to drop their price significantly, in many cases.

It would be nice, if we could somehow do a group buy in bulk thing at GOG, to leverage a good discount for a game.

That's what three to four sales/week are for, no?

Not really. Not specific enough, and many games barely get a discount or even go on sale, or it is always the same lousy discount or limited to the same games.

Buying in bulk is a very different kettle of fish ... a guarantee, rather than just have a sale and see how many bite.

Where's the "bulk" involved when it comes to:
Chasm: The Rift?
Diablo + Hellfire
Ignition
Metal Gear Solid
PowerSlave Exhumed
Railway Empire - Japan
SoF: Payback
The Witness
Warcraft I + II Bundle?

Usually, "buying in bulk" means "buying many of the same items".

Edit:
Your idea of "buying in bulk", is to buy several games from different publishers, but basically demanding from any of these different publishers that - because you, at the same time, also buy from their competitors - THEY ALL have to lower THEIR prices a few percent further, to make your "bulk purchase" more enticing to you.

Question: if you were one of these publishers...would the above demand sound like a reasonable proposal to you?

Well!, they are all games. So, there's that... :P

Distributed by many different companies. There's that. ;o)

The Last Spell looks potentially interesting, waiting for discounts.
The Last Starship is cool, but I felt it wasn't worth the asking price for a first release in Early Access. Will likely buy it again sometime down the line if they keep adding content.

Gloomwood and Graven. Exactly the kind of games I know I should like. However both were added to GOG in Oct 2020, and both are still nowhere near finished in 2023 after 2.5 years of "Coming Soon (tm)", is why I simply don't buy "In-Dev" games or pre-order anything at all...

BTW, The Witness is a great game.

Yeah, good luck with that. The last sale recorded by IsThereAnyDeal was in May 2020. That's why there's barely anything left on my wishlist here. Without a way to restrict notifications and e-mails to lower prices only, they just became an annoyance.

Dead Space - at the highest discount, it was still too much for me for a game I've already played. I also didn't want just the first game in the series. But instead of bringing the sequels over, they stopped putting this one on sale.

Dragon Age: Origins - largely the same as above, except it still gets discounts once in a while. I'll probably never buy it if they don't release the sequels here.

Life is Strange: Complete Season - I didn't want to buy it because I was hoping they'd release the remake here. Instead, they stopped putting this one on sale.

Downfall: Redux - GOGdb says it's been -80% in the past two sales, so I'm just waiting for the discount to roll around again.

Edit: typo.

Project Downfall

Would like to get this game but the dev doesn't care to update it to the latest version and so I don't care to spent money on it. Once it will be patched and up to date I'll change my attitude.

That is not my idea at all.

Note where I said - for a game. In other words buying one game in bulk .. or each game in bulk, but not simultaneously.

i.e. We'd like to buy a 100 copies of Chasm:The Rift for $10 each.

Heroes of Might and Magic I, II, III, IV and V.

I got it free at Epic, and from my quick test it is DRM-Free Lite, but I would still buy the GOG version if cheap enough.

Of course, there is some irony here, where in quite a few cases, while waiting for a decent discount (fair price) at GOG, I have meanwhile gotten a game free at Epic, even free at Steam on a rare occasion.

Yeah, I can understand that.

I have certainly considered culling games that have been on my wishlist for years, and either never gotten a discount or never a decent one.

One can fixate on the notion of only buying a game when discounted, but for me it is always about a fair price, not the fact a game has been discounted. It seems very clear to me, that a base price in many cases is set high, to make a game look good when discounted to the price it should have been all along.

The number of games at GOG that I want but haven't purchased yet...

... is too long to adequately list without dominating the thread...

... but...

... here are a few:

Knights of the Chalice 2 -- I want to support this game and developer, but 5% discounts?

Sniper Elite V2 Remastered -- how many times can one guy buy the same game? Well, the answer is "3"... if the price falls below $10 USD again.

Gloomhaven -- great game that's yet just too pricey for my blood. Very reason I don't play the board game!

Mafia trilogy -- waiting for sub $20 USD since I own them elsewhere

Grand Tactician: The Civil War (1861-1865) -- great game that's yet just too pricey for my blood

Ultimate General Civil War & Ultimate General Gettysburg -- haven't yet seen a meaningful discount

The Saboteur -- recently I found that my computer has intermittent issues with PS3 emulation, so I've started re-purchasing games from that generation... and just haven't gotten this one yet, even though it's a favorite

Banner Saga Trilogy -- just learned that I don't care for turn-based games on PS4

Kingdom Under Fire Heroes

Eador: Masters of the Broken World

Expeditions: Rome

Wildermyth -- Just not sold on the quality of the storytelling for the current discounts... yet I bought Solasta without thinking twice?

Baldur's Gate Dark Alliance 1 & 2 -- I own a ton of PS2 discs and emulate, but have wanted these on PC for some reason... but for those prices? Not yet

So, as you may or may not see from my list, I have a pretty strict monthly gaming budget... but... also have many games on multiple systems. Both of these things add up to GOG being a place where I look for deep discounts on back-up copies more than "day one" full-price purchases.