A few nice (new) features were introduced with the release of the Joomla! 3.7.x series, for example Custom Fields, improvement in the multi lingual support and the Admin menu manager. The current version of Joomla! is 3.7.4 and there are some new releases coming our way.
The first beta release of Joomla! 3.8 is already available. The most important feature for lots of users in this new version will be the new Router so now you will be able to remove those ID's in the URL's. Most of the other changes will take place under the hood. There will be a compatibility layer built in Joomla to make the update towards Joomla! 4 possible.
The main change in Joomla! 3.9 will be the completing of the compatibility layer for the upgrade towards Joomla! 4. As far as I know no more new features will be added in 3.9.
Joomla! 4 will be the new main version of Joomla and it's being developed as we speak (or read), it's still not even in the alpha stage. There are some possible improvements documented for this release:
- New core user interface based on Bootstrap 4
- Latest improvements on the new router
- SEO improvements
- Adding of a hypermedia API (webservices)
- Improvements in the install process;
- Ability to install extensions (at minimum core supported extensions)
- Rework sample data installation
- Continued work on the GSoC Multilingual editing proposal
- A new administrator template with minimal options
People are also working on a new media manager but it unclear to me if this will be added to Joomla! 4. The backwarts compatibility will be broken with the release of Joomla! 4 which can result in broken websites. This will be documented in the wiki.