I would love to see some "plug and play" user account management. Like handling registration (with confirmation emails), updating/deleting data, build in handling of changing password to increase complexity over time (or forcing change when some leak happen), some protection from login attacks, account levels (and simple adding new level over time), feature toggles, build in multitenancy (so I don't need to code it later when page grow and need it, something like [1]), using ULID (or something similar) instead simple incremental number for ID, build in login history, some login security features (like email on each login, block account login if someone try login from predefined/differen country and email user about that with unblock/allow access link) and more high level stuff (like account anonymising instead of deleting for reporting). So all the stuff that is good to have when you need it but you don't have time to work on it when you focus on business logic.

[1]: https://blog.checklyhq.com/building-a-multi-tenant-saas-data...