Let’s start by discussing what we need out of a logo and any concepts that come to mind. I think it would be really cool if someone from this community could step forward with a specific proposal, but we should focus on the requirements before judging a design by its aesthetics.
Personally, I’d favor moving away from the practice of defacing the magnifying glass in OSM’s logo. That’s a common practice among OSM’s local chapters and thematic user groups, but OHM is much more ambitious than a user group for OSM. An OSM-based logo is also nearly indistinguishable from the OSM logo at a small size, such as a favicon. If we do want to maintain a visual connection to the OSM project, we could use the OSM logo as a silhouette, such as in the Public Domain Map logo, but people who encounter OHM before becoming familiar with OSM may not intuit the connection anyhow.
I think there’s value in designing a single logo to represent the whole project, regardless of time period. Even though no single timekeeping device was used throughout all of human history, the logo can still symbolize the practice of historiography by depicting a well recognized device. Or we could go more abstract with something like a timeline.
First, the logo must be based on good practice in creating a modern commercial logo (use on white and black background, large and small version, a way to recognise the product, a recognisable logo).
I am in favour of adopting a unique logo for the OHM. The question of logos for sub-projects within OHM is one that we should not be dealing with here. The sub-projects within OHM will be free to do what they want, to use the logo they want.
Whether to continue to maintain the style of the earth and the magnifying glass depends on the relationship between OHM and OSM in its future positioning, and to what extent it will get help from OSM (such as to what extent it is allowed to use OSM data)
If it were up to me, I might choose not to use the original green map as the background of the base map, but something like File:OSM LatAm Logo.svg - OpenStreetMap Wiki or File:OSM-tw.svg - OpenStreetMap Wiki This way, use a different background
Of course, because this is a historical map, if I were to choose, I would not choose oceans and land, but something that can represent history or be reminiscent of history, the so-called “heavy feeling of history”
For example, a parchment-like nautical chart
This has me thinking of a pair of compasses drawing out a clock. It could be simple enough to work symbolically.
Another option is to use the shape of one or two famous structures as a metonym for history. For example, a representing a pyramid next to a skycraper, or something more obscure with an interesting story behind it. Or maybe a cartwheel to nod to the transportation infrastructure that we map, as long as it all doesn’t get too complex.