I'm just wondering if you have ever thought of making the outer lands as a dimension instead of overworld generation. This would allow modpacks that don't use normal overworld generation to use this mod.

This hasn't been considered. The distant parts of the overworld are usually unexplored by players, so it made sense to add a bit of exploration and reward to it. As for modded terrain generation, it's working pretty well with non-vanilla. If there's a normal overworld generation mod that isn't compatible, let me know and we can have a look into it.

Alternatively - you can turn off Outerlands in the config and vaults will spawn under normal locations in the overworld.

What about a config option for adding vaults to the nether? Perhaps as a future feature?

Vaults are generally made of overworld materials like stone, stone bricks. There are also water sources. It wouldn't be a good fit for the nether. However, I'm open to more nether ruin ideas.

I just wanted to chime in and say that I would probably use the Outerlands as a dimension. My use case is as a modpack author and server administrator. I'm unsure about how far out I want my server's world to generate, but neither am I sure I want to adjust the Outerlands distance by a significant amount. Having access to them be via a portal or similar would allow for me to gate access behind endgame materials without needing to have so much generated world taking up server space.

That said, I wouldn't say I considered it a necessity or anything. I will probably just compromise on one of my preferences in the meantime. Just wanted to add my 2c!

Outerlands has, sadly, been abandoned as a concept, so there won't be any additional changes to it.