Bitcoin has already working side chains. Rootstock (RSK), for example, allows you to simulate the Ethereum Virtual Machine (EVM), on top of Bitcoin's security and decentralization strengths. This is crazy! Layer 2 is here!

Long-term extremely bullish factor IMO.

--Edit

Info and Links:

• Rootstock website. Smart contracts glued on top of Bitcoin. Uses a 1:1-pegged Token (rBTC) to pay for "gas" and can run Ethereum contracts.
• DeFi on Bitcoin using Rootstock, exchange, lending, staking etc
• An overview of the RSK ecosystem is here (thanks /u/yr-mooning)
• Why not Layer 3?! Check out RGB which enables smart contracts on top of the Lightning Network! ????????