The first technique is from Alastair Brotchie's collection of surrealist games. The second is a variation on the first. Like all such games, it is important to play them freely, without concern about "making a good one" or being clever. If the players feel free, the results can sometimes be remarkable. We are used to trying to work together rationally, these games ask us to work together irrationally, creatively.
If/Would
In pairs or a group: each person writes the first clause of a sentence, beginning with "If", on this pattern:
"If people lived on water and air alone," or
"If my head were a watermelon,"
They then fold over their papers so that the clause is not visible and exchange papers. Step two is to complete the sentence by writing the "then" clause:
"...then all my dreams would come true." or
"... then peace would reign on earth."
When all are finished, each person unfolds her/his paper and reads the finished sentence, e.g.:
"If people lived on water and air alone, then all my dreams would come true." or
"If my head were a watermelon, then peace would reign on earth."
Note: Obviously, you can drop the word "then" but it helps to keep the idea of the game clear.
Variation:
When you have several sentences made, tear the paper into strips, so that each clause is on a separate strip of paper. Gather them into two piles, one for the "ifs" one for the "woulds". Take turns selecting a strip of paper from each pile and reading the resulting sentence.
When/Will
The same game, but this time the pattern sentence is "When ....., .... will....." Example:
"When the will of God is done on earth as it is in heaven, there will be no more marriage." (J. H. Noyes)
Players write "when" clauses, conceal them, swap papers, and write "will" clauses. E.g.:
"When the world is obliterated by a meteor,"
"When all my dreams come true,"
"When every factory is under worker control"
"...cats and dogs will lay down in peace."
"... gender will have no more meaning."
"...I will never eat yogurt again."
This version is more declarative -- you could follow up with an improvised sermon based on one sentence.