I was looking for a new extension for phpBB 3.1, too. As I did not find anything, I reuse the data given (the location database) and collected (custom user fields) by the old GeoMap MOD for phpBB 3.0. Out of this data I create a Google Maps map using a bit of PHP and the Pages
So, see here, I like it: http://www.sidemount-forum.com/page/usermap
As the old database tables of GeoMap are still in the database after upgrade and also the user is asked for his/her ZIP code and country, new users are also listed in the map as well.
Problem, it is nothing which can be distributed as a new extension and I do not have the time for it. If someone has more time and also programming skills I am happy to share the files. I would love to see a new extension which can be downloaded, activated and managed in ACP. Maybe also an extension which is capable of more than locations in Germany, Switzerland and Austria. Therefore the database solution must be changed into something else. No idea...
Everything is possible. But as I said, I am only a hobby programmer and the thing I created is good for my project - at least for the moment.