UPDATE:

We have now released Odoo versions 15.1 and 15.2, and you can see What's new below:

ORIGINAL ANSWER:

We plan to release Odoo version 16 during Odoo Experience in October 2022.

Disclaimer: Please note that a product roadmap is a forward looking vision subject to change based on many factors, including market conditions; resources; technical challenges; and other factors that may or may not be in our control. This answer is not a commitment to deliver features. I am sharing the "at the moment" priorities. Remember that "The best-laid plans of mice and men often go awry".

Also, this is not an official Odoo statement.

• NEW APP "Knowledge" that centralizes an organizations information and best practices (inspired by Wiki, Notion and our internal Odoo knowledge base) that is integrated with Chatter and can be accessed while completing workflows.
• Manufacturing Worksheets (like Field Services) to help Users more easily complete/document operations, steps and quality checks.
• Merging Sales Orders and Subscriptions to simplify management.
• Refactored coupon/loyalty logic to support more use cases and unify the approach across eCommerce, Point of Sale and traditional Orders.
• Enhance our Call for Tenders feature in the Purchase App (no longer need to decide in advance).
• Simplify setting up eCommerce - minutes to sell and receive payments online. 
• Improve mass mailing features.
• Improvements to payments (including ability to refund Customers from within Odoo).
• Evolution of the user interface.
• Improve the user experience of localizations.
• Continue the upgrading of our web UI framework to OWL and iterate OWL itself to v2. Learn more at https://github.com/odoo/owl
• Continue to optimize performance and refactor code where we can simplify it and reduce lines of code.
  

Please do not make any decisions (or assumptions) based on this information until version 16 is released.

You can monitor our work on version 16 at https://runbot.odoo.com (the branch MASTER is our current branch and saas-15.X are the point releases that will comprise the version 16 release).

This information is mainly from our 2021 All Company Meeting which you can watch for yourself at https://www.youtube.com/watch?v=iDzzo7zdgFo&t=2318s