I come from the Wabears fandom, where Forum Games are a staple of life.
So I'll start with one of the simplest ones out there:
Ruin A Wish
First, someone posts what they wish for. For example:
First person: I wish for a cake.
Then the person that posts after them ruins the previous wish, and does their own.
Example:
The cake falls from the sky and hits you in the face!!11
I wish for a new chicken.
(colors not needed)
The cycle then continues, theoretically, forever. Only one post per day please.
I will start:
I wish for a T.A.R.D.I.S.