I've listed few projects that are both easy to build and easy to extend with new features.
/* Project 1: Chat App */
A chat application, some sort of stripped-down Slack, where users can enter without authentication needed. They App will then assign users a name which will be stored and saved so they can come back to the same account. This will need some sort of history store, messages system, and notifications system.
/* Project 2: Book Database */
A database where you can store your own books (containing books info and images), having the possibility of creating a list of books you'd like to read/buy.
Also you could add a book reviews system where other users can type their thoughts on the reading (similar to Codú articles comment).
/* Project 3: Recipes App */
An App that allows you to create a new recipe (name, steps, description, images/video) and will also rank recipes according to its difficulty and/or time required.
This will also involved a database where recipes are stored.
/* Project 4: Pomodoro App */
It will involve users setting specific time, a start button, and an alert in place when the time is over.
/* Project 5: Q&A system*/
A QA system that will allow user to login, ask/answer questions as well as ranking best questions/replies made by other users.
In this case, data should be stored in a database.
Check out this video for more ideas: freeCodeCamp.org tutorial