Youtube creators already have the ability to create series inside playlists.

Example: The Slow Mo Guys channel has a "Planet Slow Mo" series: https://www.youtube.com/playlist?list=PLbIZ6k-SE9Shmj0Uvxtrv...

The problem is when you first find a video that's interesting you don't necessarily find the playlist that it belongs to. (or worse, you find it in someone else's playlist which includes the video along with other seemingly random videos that the playlist creator was interested in)

This is not available to... well, almost anyone. Seriously try searching for it. Every permutation of your search terms will bring you back to one of three results that all heavily imply or say that it's available to a select handful of channels

It's not even subscriber gated or verified channel gated, it's someone at youtube for some reason flicked the switch on for this channel gated

Sometimes they're in a playlist, but sometimes I'm not watching via the playlist. Like if I know that episode 19 is the next one I need to watch, I go in search and find "Super cool series episode 19", and then I watch it.

I don't search for the playlist, then go to the playlist and scroll through to the episode I want, then pick it.

But then since I didn't go to the playlist, now YouTube can't recommend the incredibly obvious next video in the series.