i propose that each post has a (one or many) geo_tag fields that just link to the canonical wikipedia page about the place, for example,

{
  "post_id": 1543535,
  "geo_tags": [
    "https://en.wikipedia.org/wiki/Melbourne"
  ],
  ...
}

I doubt Lemmy will ever do this - e.g. moderator reports still don’t federate to other instances until the release of v1.0, despite the Rexodus having been several years in the past now. Basically any solution would have to be on top of the software without needing any changes within it. Lemmy puts up full-page advertisements for donations but a lot of that funding goes to running Lemmy.ml and seemingly only very little to actual code development.

Although predictably PieFed already has this functionality, for well over a year now. For one it has hashtags, plus user and post flairs, and for another it has categories of communities where someone can look at e.g. news across all regions, or pick let’s say Europe and then choose from various sub-topics below that. Also, while these Topic areas are defined by the instance admins, the otherwise identical concept of topical Feeds are user-customizeable and even shareable, so someone has likely made what you are looking for already, but if not then you could make it and share with others to benefit from your efforts.

At worst even, say when interacting with existing communities that did not want to actively participate in the process of self-sorting their own content, users have keyword filters that can be used so that you as an end-user can do it entirely on your own. Such community discovery and management concerns are a solved problem on PieFed. I say this full well as someone who had the identical issue you described here when I was on Lemmy, and moving to PieFed solved it for me.

The developers are also extremely receptive to feedback, if you needed still more changes made to the code. I sincerely doubt that you will ever get a solution going using Lemmy - this identical (edit: general) concept has already been asked for many times over the years - but switching to PieFed should easily take care of it, offering multiple possibilities to make finding the content that you want easier.