"How would your strategy be different if the goal was to get a modest return in a stagnating market, a larger return in a market crash, and a loss in the event of sustained growth? Do you think there is a way to guard against transient bubbles?"

Well, you could supplement the strategy with out of the money puts, that would pay more farther down to the downside, but you'd be really limiting your return with that. In fact, puts are so expensive right now that adding options that give you just a 200% return on this position when the S&P 500 hits 300 (this would be amazingly bad) drops your original stagnation position's returns to -20%.

I'll do a more detailed strategy and see what kind of returns we can create to cover the "way down" side.