First and foremost, blockchains need to address the scalability issue.
What do we mean exactly by scaling in the context of blockchains? A core security feature, but also a capacity limitation, of public blockchains such as Bitcoin and Ethereum is that their protocols require every transaction to be processed by every single full node in the network. Every operation that takes place on the blockchain, such as a payment or a deployment of a smart contract, must be replicated by all full nodes. This is what makes public blockchains secure, autonomous, and decentralized at the same time. Participants don't have to rely on someone else to tell them what the current state of the blockchain is; they figure it out for themselves.
This puts a constraint on a blockchain's transaction throughput. It cannot be higher than the processing capacity...