Federated application for a map with markers and notes?
It seems for me that this would be too narrow a purpose.
Maybe a general-purpose public notification map. With some functionality allowing to separate markers by their authors and by tags. Or it can be spammed with bogus markers. By tags - well, for it to be general-purpose. By authors - because moderation can't be left to instance admins.
And, of course, I'm personally for separation of moderation, instance ownership, identities and hosting, but my own toy attempt showed me that the logic of checking the chain of privilege delegation is kinda PITA. That is, separating identities from instances is not that hard. And communities. What's hard is the community owner delegating rights to other identities, and in general authorized actions. It's a task of determining which privileges does an identity currently possess, and how does it affect its own actions on the community, and in which order should those be processed ... Everything is harder than it seems. Sad.
So federation is fine LOL.
Sorry for my tone too, I get dysphoric-defensive very easily (as have been illustrated).