For the first case of WorldBorder, there are actually a few plugins which can handle the borders and the generation features of the plugin. The essentials plugin Nucleus if you are using that has some great border and generation functionality that has been highly tested. If you are using a different essentials plugin or not looking to use one at all, you can check out Project Borders which is a plugin with just border and generation functionality built in. Unfortunately neither of these support circular borders due to them both utilizing Minecraft’s built-in border functionality, but that works for most people.
You are correct though that there is no Giant Trees equivalent at the moment, so sorry about that. There are some interesting Forge mods focused on generation that could be worth considering though such as Realistic Terrain Generation or the Biome Bundle which relies on the TerrainControl mod but that has had some issues and the mod is not really updated much anymore.