I especially like the theory of the Old Ones being stranded. Imagine they get to the Warhammer world by accident, then as suggested in that thread just do what they think is best for survival. Then, having knowledge of the Warp and how it works, they create races capable of thought and emotion, thus feeding the Warp, and thus Chaos. Then when the Warp is churning with energy, they build the Warp Gates and use them to get the hell out of here, leaving the world flooded with Daemons.
And then, as I like to tie in the Fantasy Universe with the 40k Universe, they use the same blueprints they used for elves and men on other planets because they saw the potential of these races.