Messing with the mining difficulty with the intention of cancelling out what you were doing later wouldn't do much to deter speculation, because speculators would price in the fact that the difficulty change was going to get cancelled out.
The difficulty change would get cancelled out when the price went back to target levels. So if speculators priced in this fact, there would be no bubble in the first place and the price would remain stable. Ideally the difficulty would correlate with the rate of change of price: the faster the price (or leading indicator) is rising, the easier it is to mine coins. The faster the price (or leading indicator) is falling, the harder it is to mine coins.