Yep, options would be simplest. CFDs are similar to options and should be fairly simple to implement. With options you just have a buyer or a seller, while with CFDs and futures you need both, so you’d need an exchange or marketplace like localethereum.com. Although with options you still need a way to actually execute the option which could be done on an exchange, but there is just no counterparty to the trade.
As others have pointed out there are other derivatives that are more complicated that have been implemented in non-crypto world as well as prediction markets, and others that AFAIK haven’t been implemented like inverse instruments.
When you have a counterparty they have to take on the potential risks and rewards, which retail investors and institutional investors may not want to do given the volatility of cryptocurrencies.
There are others who may buy up a lot of a security then sell it, e.g. as done with George Soros and the pound. This then has a flow on effect of others reacting to the price change. But to have this effect you need a lot of capital. However, AIUI cryptocurrencies are more sensitive to this effect as there is liquidity can dry up if you sell too much, which happened with the GDAX flash crash. Then orders are triggered which causes much more capital to be sold. Omega One is looking to provide a solution for this.