Roadmap for core Media integration

This is a followup for #2999198: Roadmap for core Media integration related to Slick and core Media integration with oEmbed.
As I don't follow core Media closely, I would appreciate any suggestion from those who follow core Media closer.

There will be pain points with these changes.
Please help identify any either in your comment, or feel free to update this summary.

Problem/Motivation

Blazy 8.x-2.x now depends on core Media with oEmbed. This also drops supporting VEF/ VEM in favor of oEmbed. Decide what to do with the following contribs:
https://www.drupal.org/project/slick_video
https://www.drupal.org/project/slick_media

Since Drupal has made file and image as legacy fields in favor of Media (when you add a new file and image, Drupal warns you mentioning they are now legacy fields). The above two contribs are now redundant. Redundant as Media integration is no longer optional. The main reason for the current separation is optional integration.

The main benefit I can see is reducing the other two separate modules to maintain, or worry about.
And for users, less modules to download and enable.

Remaining tasks

TODO. Let's list thing here. If anyone has been more involved than me with core Media recently, please update, and remove this line.

• #3023234: Create a new branch for core Media
• #3023240: Add a new Media oEmbed base formatter
• #3023353: Implement the new Media oEmbed base formatter
• [X] #3023355: Please help decide the future of this module
  Irrelevant as Slick Video is intended for VEM only.
• #3023354: Please help decide the future of Slick Media

What replacement?
Likely a new Slick oEmbed, or just Slick Media (with oEmbed) which shall be included in the main module. Alternatively simply let them both as they are now with/ without oEmbed support? Or provide a hook_update() to migrate into main module as a plugin.
Detailed info: #3023353: Implement the new Media oEmbed base formatter

User interface changes

None. Exactly the same.

API changes

None.

Data model changes

None. VEM already provides upgrade path: #2997799: Include upgrade path from video_embed_field to oEmbed

Release notes snippet

None.

Known issues

Core oEmbed has little room to extend/ modify as of this writing. This causes two known immediate issues which require solutions beyond regular JS or CSS overrides, some extra leg:

• [x] Non-responsive video due to iframe within iframe thingies.
  #3022743: Create a new branch for core Media
• [x] Clicking custom play button with media switcher "Image to iframe" will fail:
  #3023345: Image to iframe doesn't autoplay oEmbed video
• List other things here. Feel free to update.