Theoretically speaking, could one "predict" what games are more likely to go on a Daily Deal duriing the Winter Sale by looking at which ones are not discounted right now, but "should"? For example, Titan Quest was not discounted (but its expansion was), then it became a Daily Deal.

& by looking at promo store page and noticing Sundered hasn't been discounted despite featuring as promo.

No love for Incredible Machine Mega Pack, GoG?

Daily Deal
15.12.2018
Winter Sale 2018 edition

Dream
$16.99 --> $1.69 [90% off]
New best historical price

For The King
$19.99 --> $6.99 [65% off]
New best historical price

Herald: An Interactive Period Drama - Book I and II
$9.99 --> $5.99 [40% off]
New best historical price

Immortal Redneck
$19.95 --> $4.99 [75% off]
New best historical price

Blood and Gold: Caribbean!
$19.99 --> $2.99 [85% off]
New best historical price

Crimsonland
$13.99 --> $3.49 [75% off]
Not the best historical price

Solstice
$19.99 --> $4.99 [75% off]
New best historical price

Kingdoms and Castles
$9.99 --> $6.99 [30% off]
New best historical price

Dream base price is 14,99€ on GOG,
12,99€ on Humble Bundle and 6,99€ on Steam.

I guess that's the price we pay for DRM-freedom. :p Note that Humble Bundle version is not DRM-free, it's Steam key only.

Daily Deal
16.12.2018
Winter Sale 2018 edition

Prince of Persia
$9.99 --> $2.49 [75% off]
Matches best non-bundled price

What Remains of Edith Finch
$19.99 --> $7.99 [60% off]
New best historical price

Prince of Persia: The Two Thrones
$9.99 --> $2.49 [75% off]
Matches best non-bundled price

Where The Water Tastes Like Wine
$19.99 --> $9.99 [50% off]
New best historical price

Prince of Persia: Warrior Within
$9.99 --> $2.49 [75% off]
Matches best non-bundled price

MachiaVillain
$19.99 --> $9.99 [50% off]
New best historical price

Prince of Persia: The Sands of Time
$9.99 --> $2.49 [75% off]
Matches best non-bundled price

Defender's Quest
$14.99 --> $5.99 [60% off]
Not the best historical price

When the winter sale started I sp[specifically searched for Vaporum, and it wasn't on sale. Yet today it is. Is it part of the daily deals and I'm just not seeing it? Or is GOG adding games in the background to the sale, which makes it so much more difficult to have an idea about what to buy.

GOG set 8 games as Daily Deal per day during Winter Sale.
At the same time, they add new games to the sale randomly.
Since Vaporum is not Daily Deal, I guess its discount will lasts till the sale end.

Exactly what kbnrylaec said, Vaporum was added to the regular Winter Sale earlier today (day 5).

Vaporum
$19.99 --> $9.99 [50% off]
New best historical price

--------------------------------------------------------------------------------------------------------------

Daily Deal
18.12.2018
Winter Sale 2018 edition

My Memory of Us - Collector's Edition
$19.99 --> $11.99 [40% off]
New best historical price

STAR WARS - The Force Unleashed Ultimate Sith Edition
$19.99 --> $4.99 [75% off]
Matches the best historical price

STAR WARS Battlefront II (Classic 2005)
$9.99 --> $3.49 [65% off]
Not the best historical price

ABZU
$19.99 --> $7.99 [60% off]
Matches the best historical price

Planet Nomads
$19.99 --> $8.99 [55% off]
Matches the best historical price

Seven: The Days Long Gone
$29.99 --> $8.99 [70% off]
New best historical price

The Signal From Tolva
$19.99 --> $4.99 [75% off]
Matches the best historical price

STAR WARS: X-Wing Alliance
$9.99 --> $2.49 [75% off]
Not the best historical price

Bump, what about the new daily deals?

Daily Deal
19.12.2018
Winter Sale 2018 edition

Lake Ridden
$19.99 --> $7.99 [60% off]
New best historical price

STAR WARS: The Force Unleashed II
$19.99 --> $4.99 [75% off]
Matches the best historical price

Way of the Samurai 3 - Deluxe Edition
$24.99 --> $4.99 [80% off]
New best historical price

STAR WARS: Knights of the Old Republic
$9.99 --> $2.49 [75% off]
Not the best historical price

Xenonauts
$24.99 --> $3.79 [85% off]
New best historical price since offered free

STAR WARS: X-Wing Special Edition
$9.99 --> $2.49 [75% off]
New best non-bundled price

Depth of Extinction
$19.99 --> $12.99 [35% off]
New best historical price

STAR WARS Jedi Knight: Dark Forces II
$5.99 --> $1.49 [75% off]
Not the best historical price

Daily Deal
20.12.2018
Winter Sale 2018 edition

STAR WARS Knights of the Old Republic II: The Sith Lords
$9.99 --> $2.49 [75% off]
Not the best historical price

FAR: Lone Sails
$14.99 --> $10.49 [30% off]
New best historical price

STAR WARS: Dark Forces
$5.99 --> $1.49 [75% off]
Not the best historical price

Overload
$29.99 --> $17.99 [40% off]
New best historical price

The 25th Ward: The Silver Case
$29.99 --> $10.49 [65% off]
New best historical price

Urban Empire
$29.99 --> $5.99 [80% off]
Matches the best historical price

Not Tonight
$19.99 --> $13.99 [30% off]
New best historical price

Pandora: First Contact - Gold Edition
$31.99 --> $4.79 [85% off]
New best historical price

Does the DLC pack for the 25th Ward on sale here include everything the LE pack of the game does? It would seem to but my concern is the OST on MP3 with the DLC but both FLAC and MP3 with the LE? I mean the OST listing for the LE doesn't explicitly list the format.

New Daily Deal
21.10.2018
Winter Sale 2018 edition

Polaris Sector - Gold Edition
$44.99 --> $8.99 [80% off]
New best historical price

Mary Skelter: Nightmares
$24.99 --> $13.69 [45% off]
New best historical price

Kromaia
$19.99 --> $2.99 [85% off]
New best historical price

STAR WARS: X-Wing vs. TIE Fighter
$9.99 --> $2.49 [75% off]
New best non-bundled price

Songbringer
$19.99 --> $6.99 [65% off]
New best historical price

We are the Dwarves
$24.99 --> $1.29 [95% off]
Not the best historical price

Deep Sky Derelicts
$19.99 --> $6.99 [65% off]
New best historical price

AI War Collection
$16.99 --> $3.39 [80% off]
Not the best historical price