I think some things should be implemented in the spooky dimension. Only personal opinions.

• Generate a new Spooky River biome instead of the overworld rivers
  • No squids anymore
  • No overworld grass blocks on the shores anymore
• Fix the crash when we try to generate vegetal features in the dimension
• Once the above is done, add the following features:
  • Pumpkin patches
  • Cobweb patches, with a chance of generating a spider/cave spider spawner in the middle
• Replace the hostile mobs by new spooky mobs and/or customized vanilla mobs

I think it would be cool to have candy goo & witch water rivers. Candy goo is bright green and slows you down. Witch water is purple and has a bubbling particle effect. When a mob walks into the later, they may be transformed (eg. villager -> witch).

As for pumpkin patches, I also think we should have a pumpkin field biome. Would be cool to have little pumpkin guys wandering around with a giant pumpkin boss.

There should be at least 3 biome groups imo
Some ideas for biome groups but these could definitely be refined:

• One [biome group] for dense forest and swamp
• One [biome group] for hills and grasslands

I feel like biome groups should be distinctive by color, look, and feel.

Or maybe forests and swamps could be in separate biome groups, who knows

Not everything should be super dark and gloomy like the current biome, maybe have a certain biome group which is a bit happier overall.

But we should definitely have swamps and marshes