Would providing your own GEOJson maps work? It also seems to be an x-pack feature, and it would be a great option to be able to just upload a custom GEOJson directly to Kibana. For example, I am trying to develop a homegrown COVID tracker for myself, using data from the regional government and a GeoJSON map of the region. Supplying this info to Kibana and generating a map visualization just off the GeoJSON would be a good option for me. (I do understand that the GeoJSON can’t be too big, but for small regions, I don’t think it should be a problem)
PS.: If providing a default external maps provider is not a viable option, a built-in simplified “referece” map as a base layer or for testing/debugging purposes would be very nice (if it is not available right now).