This would be very useful in displaying vehicles used by multiple groups.
I agree with many other comments on this page. There are many workarounds and adding multiple parents to one item would most likely cause many issues.
Try using tagged lists, they're great for this purpose! Here's a link to the guide on how to use them: https://www.worldanvil.com/learn/bbcode-tutorials/tagged-lists
It would be great to be able to put an article in many places in the TOC. I've missed that many times. Personally I would love to have multiple TOCs too. But these needs have to b solved in other ways by linking and using tags, and I believe the team made it clear that this is the way it will work.
This is probably impossible to implement without a huge change to the WA database structure, the way that linking works, and the dashboard and article tables of contents work. I can understand why it might be useful in some cases, but in reality, what you are asking for is a major rewrite of the WA software.
I think this sort of thing is better accomplished with other methods already existing in World Anvil. (Some of which are already mentioned in other comments on this page.) Multiple parents is problematic. For example, breadcrumbs. (It could be confusing to readers if the breadcrumbs change depending on how you got to an article.)
The way the tree hierarchy works is that an article have multiple children but only one parent. Similar to how articles can only belong to one category, and a category can have many article and subcategories. This is actually a great use case for tags where deities can be tagged with all the pantheons they occur in. Cities can be assigned an owning organization (country) and location (region) already.
This is an interesting topic which is perhaps too complex to address in a short response but my thoughts are that having a simple "preferred" hierarchy in which any article can be in one place and one place only, has a conceptual simplicity which not only makes it easier to manage functionally but easier to think about. There is a reason why the tried and tested "folders" and "files" paradigm in file operating systems (e.g. windows) continues to hold sway despite the overlay of "libraries" & etc... That said, of course it is essential in a wiki style presentation like WA that we can reuse our articles & group them and present them in multiple different structures. Your example of deities in multiple pantheons is the kind of thing I want to do all the time. There are different solutions for this already, though. You can, for example use article blocks to make different parent pages for the same articles (or different subsets of the same ones). These look pretty and from the perspective of the reader they do not privilege one "parent" over another. I have cities grouped under a continent and then grouped again under the empires or kingdoms that currently control them, for example. However, this doesn't work for the tree view in navigation and I do accept that is a limitation of the current design (but one I am prepared to live with, because I can make pages using BBCode that give me 99% of what I would want from multiple parenting anyway).
This is a very rare occurrence - I would really prefer we do not litter the UI more with more options we hardly ever will use
I imagine this would be difficult to accomplish, but it would be pretty useful for users