Idea Summary

View Transitions API has been available for a while now. It is supported by all major browsers (except Firefox), Safari will fully support it soon (currently supported if Experimental Features are turned ON). What View Transitions can give the APEX apps is a native feel, as it will allow different regions on the page to appear on screen in an animated way, as well as switching between pages. It is currently possible of course manually, but it will be nice if these were declarative.

Resources:
https://developer.mozilla.org/en-US/docs/Web/API/View_Transitions_API
https://developer.chrome.com/docs/web-platform/view-transitions/
https://http203-playlist.netlify.app/ 

Use Case
Whenever users want to give their apps (especially ones optimized for mobile view), it will be far easier if there was a predefined list of transitions that could be used declaratively - both on page and region level.

Preferred Solution (Optional)
Add a new Option in Page/Region Templates which is called Transition. Once selected, it will animate the element when it gets opened (or closed, depending on how complex the implementation will be).
Sample: