I hope that Pyke is revealed to be an undead not tied to the Shadow Isles in any way.

In both the old and new lore, we have had stories about undead/necromancy that exist separately of the Shadow Isles. In the old lore, a necromancer wounded Irelia and inflicted Trundle and his tribe with a plague (RIP real Trundle btw, you are missed).

In the new lore we know that Mordekaiser is the first of the dead from the Isles, using dark magic to bind his soul to his armor with his body as an anchor to the world. He gains power from the Isles because their ability to dominate souls pairs well with his, but he was originally an undead with liches long before the isle of the dead existed.

The one thing I've found puzzling is that Riot has never really added an undead champion off the Isles, but hopefully Pyke will be the one to fill that Void. It is possible that he was claimed by the Mist, but I think the Swain quote "They fight the mist above the seas, never looking beneath" might show that the threat is separate, although it could also imply they never thought about what the Mist does to marine life around the island which could include Pyke's burial place.

The giant sea monster that ate Pyke is tied up, which leads me to believe that Pyke wasn't just betrayed by his crewmates- he was sacrified. Perhaps the monster is some sort of god, possibly even Nagakabouros, and is trapped there as a nuclear option to defeat the Isles (like how Illaoi does in Shadow and Fortune). If it is some sort of god and clearly has some connection to spirits as seen with Illaoi, it stands to reason that this creature could've resurrected Pyke for the purpose of enacting revenge on Bilgewater. It is yet to be seen.

TL;DR, I just think there's a lot of thematic space for an undead not directly tied to the Isles, we know it can happen and it could get into some of the deeper mythos of the League universe which is always something that gets two thumbs up from me. Sion and Mordekaiser are the only real examples so far, but Sion is barely sentient and Mordekaiser is awesome but there's room for more.