D&D 5E (2024) Feywild technically doesn't mention the Seelie and Unseelie Court

[MechaTarrasque]

At least some of the fey are created by strong emotions (Red Caps come to mind), so in theory the "positive" emotions (or the ones people generally desire to experience) could be one court, and the "negative" ones (or the ones people generally desire not to experience) could be another court. Of course, too much of a good thing can be bad for you...

I tend to play up the melodrama when my campaign enters the 'Wild.

Since the Shadowfell is about suppressed emotions, I treat fey there as a kind of undead, trying to eat the emotions of other beings (psychic damage and doing damage to Cha scores instead of regular damage/other attribute damage). If a 'Fell fey kills a 'Wild fey, the 'Fell fey goes back to being a 'Wild fey and the former 'Wild fey wakes up as 'Fell fey 24 hours later. Non-fey are not as useful, but if a 'Fell fey kills a PC (after making them feel a strong emotion), that will probably be enough for them to change back to 'Wild fey.

 

[Corinnguard]

Fey are born from the ephemera of dream, and thus are fickle and capricious creatures, capable of turning from mirth to malice in an instant. Fey have no fixed nature and thus must live by their ideals and their oaths lest they unravel themselves. Nonetheless different fey may find in any of the courts or some will change courts according to the seasons, mood or personal whim.

Level Up has it where the Fey are members of the Dreamborn heritage. The Feywild in Level Up is called the Dreaming.