Games like billion
Games like billion include Bongo Cat, The Murder of Sonic the Hedgehog, Cell to Singularity - Evolution Never Ends, To The Core, and Coloring Game. We matched them by the tags players give billion on Steam (Casual, Point & Click, 2D, Old School, and Retro): closest matches first, and among equally close ones the better-known games.

Bongo Cat
Shared: Free to Play · Casual · 2D

The Murder of Sonic the Hedgehog
Shared: Point & Click · Casual · Free to Play

Cell to Singularity - Evolution Never Ends
Shared: Casual · Free to Play · Incremental

To The Core
Shared: Casual · 2D · Incremental

Coloring Game
Shared: Casual · Free to Play · 2D

Purple Place - Classic Games
Shared: Casual · Retro · Old School

Papers, Please
Shared: Point & Click · Singleplayer · Retro

That's not my Neighbor
Shared: Casual · Point & Click · 2D

Samorost 1
Shared: Point & Click · Casual · 2D

Idle Champions of the Forgotten Realms
Shared: Free to Play · Incremental · Casual

Nodebuster
Shared: Casual · Incremental · 2D

The Tiny Bang Story
Shared: Point & Click · Retro · 2D

Digseum
Shared: Casual · Incremental · Point & Click

Cookie Clicker
Shared: Casual · Incremental · 2D

Transformice
Shared: 2D · Casual · Free to Play

Broken Sword 2 - the Smoking Mirror: Remastered (2010)
Shared: Point & Click · Retro · 2D

Varenje
Shared: Casual · Free to Play · Point & Click

Click the Button
Shared: Casual · Point & Click · Incremental

There Is No Game: Wrong Dimension
Shared: Point & Click · Casual · 2D

Storyteller
Shared: Point & Click · 2D · Casual

My Brother Rabbit
Shared: Point & Click · 2D · Casual

Rhythia
Shared: Casual · Point & Click · 2D

Kaze and the Wild Masks
Shared: 2D · Old School · Casual

Shelldiver
Shared: Casual · Incremental · 2D