If multiple playlists are never going to happen, then how about a selectable tag system for individual episodes?
The Categories filter only applies to whole podcast feeds, but many of those have hundreds of episodes and lots of people like to pick and choose, and group them for different listening situations. Multiple custom playlists is the most obvious solution, with hundreds of votes here, but it doesn't seem that's happening.
So what about the option to create tags that can be applied to and used to filter individual episodes? Those can be added to the custom playlist or played from whichever list they're filtered in. It would be more work, but also potentially more flexible than multiple playlists, since ideally you could filter by more than one tag, like *kids + *car + *weekday. Although most would just use it like *workout or *bed.
*nsfw
*sleepy
*motivation
*business
*business time
*ichthyology
*comedy