Regulation.
Stay with me, I will explain. Regulation will need more investors, the more investors can buy bitcoin . Not some derivatives backed by BTC price but hard core Bitcoins. If there is more investors, there are money in market, the more money in market - less volatility.
I do not think that the number of investors in the market will somehow influence the level of price volatility. Today they are investors, and tomorrow there will be sellers. Demand and supply will change strangely, and therefore the volatility of Bitcoin and other decentralized cryptocurrencies will always remain. This is a huge advantage and at the same time a huge disadvantage of cryptocurrency.