Some will stay pretty much exactly the same, but then compatible for Joomla 4.
Some will be completely recoded and be available in a different form (not fully backwards compatible).
Some will be combined (Tabs and Sliders).
Some will not be useful enough for Joomla 4 and will not be made available.
I see that on your blog post Joomla 4 Plans and Roadmap you mention that Extension Manager for J4 will be released by the end of the year. Is this still on track? And about the extensions that are already ready for J4, do they install the Regular Labs library by themselves?