Postgres

Currently there's no way to easily write a nice client-side app, fully decentralized app with similar ease that you would in Web2. In Web2, you spin up a database on an AWS instance, and you have your client-side app call this database to read and write. There is no equivalent in Web3.

You can't just write the data into Ethereum, which would be too expensive for most use cases. Storage protocols like Filecoin and Arweave are primarily designed to archive data, but provide no enterprise-grade performance guarantee for writing and reading data.

A decentralized Postgres would enable a plethora of new applications.