I was wondering, can you make a custom vacation world with tombs, base camp, etc? I know that Nraas Traveler is a thing, but isn't there a way to make the game recognize a custom world as a travel destination on it's own (without the mod or replacing the existing worlds)?
I'd love to make my own vacation world that feels just like the premade ones, including:
- lots such as tombs, a base camp, market
- local food (I know how to make a custom dish, but is it possible to make the food and the recipes for it appear only in the world's market?)
- custom loading/map view music
- custom loading screen title (Travelling to <name>, <country>)
- new relics
- new opportunities
- visa levels with custom titles

Is any of this possible?

Making the game recognise a custom world without Traveler would essentially be... scripting your own version of Traveler
It wouldn't be possible without script modding and would by and large be achieving what already exists in that mod.
EA's travel scenarios are extremely broken and I would 100% recommend NRaas Traveler to anyone doing any kind of world travel (including University and Oasis Landing) because it fixes so many of those issues. With that in mind, embracing Traveler as a requirement seems a good place to start.

There are many custom worlds designed to played in that way. Tombs and base camps can be made in any world, and the latter will automatically be where the sims arrive when visited with Traveler.
Expanding anything tied directly to one of the three WA presets may not be impossible, but would require a significant amount of script modding, if not core.

Local food would likely involve making a custom register script, for custom loading you could look into how Gamefreak130's Loading Screen Overhaul works.

Short answer, possible with a lot of advanced work but significantly easier if the world is simply designed to be visited with the existing mod. Doesn't mean it can't be full of functionality and unique details that bring it close to the experience of playing World Adventures, but there are limits within reason.

There have been posts on this before including where I tried to make one, and IMO CardinalSims is totally correct to say it is best to just make a world and have Sims visit with the Traveler mod. In the world I did other Sims came but all left at 4 pm. I think a service person or 2 remained, but other than them and my one test Sim the world empties then.