Re: Property keys unique per label
Thanks for explaining in more detail. Another example is a "color" property. Different data sources could use different types of color objects. As long as you do not want to query for paints and socks with the same color, there is no real need to harmonize the color data-value types.
Also note that an index on a property can already be constrained to a single vertex or edge label. So, if anyone would contribute your idea as a JanusGraph feature, I would guess there would be no objection.
Best wishes, Marc