Next giveaway suggestions?

Maybe something a little bit less known:
Lethis - Path of Progress
1979 Revolution: Black Friday
Shovel Knight
Parkitect (not sure about the US price)
The Sea Will Claim Everything
Don't open the doors!
8-bit Invaders!

Whole humble monthly =))

Just Cause 3 and Fallout 4 are both on sale for less than $15 atm (fallout 4 steam key from another website tho, can't remember the name tho), but for now I'll just say Bioshock Remastered :) really liked the game myself ;)

Pillars of Eternity ++++
SEGA MEGA DRIVE and Genesis classics +++
Walking Dead series ++
SOMA +
Splinter Cell series +

Ori and the Blind Forest, that one would be amazing, normally it costs 20$ but it is under a discount atm.

Stardew Valley is pretty popular.
Subnautica might be in this price range but I do not remember for sure.
This is the police
Orwell
KONA
Rocket Leaugue when on sale

Humble Monthly is a good deal probably.

If you just want clicks as possible reach, then just look through the community wishlist and try to find something affordable there. Yeah, it is mostly just a list of "highly marketed game of the week", with AAA titles gradually fading into oblivion as the next game nobody actually wants to play/finish comes out, but they can generate interest.
Skyrim and Rust are usually good choices though, as they are relatively popular plebs games as well. You cannot really go wrong with them.