To some degree, metropolitan regions owe their existence to the ability to valorize agglomeration economies. The general perception is that agglomeration economies increase with city size, which is why economists tend to propagate urbanization, in this case in the form of metropolization. Contrarily, spatial planners traditionally emphasize the negative consequences of urban growth in terms of liveability, environmental quality, and congestion. Polycentric development models have been proposed as a specific form of metropolization that allow for both agglomeration economies and higher levels of liveability and sustainability. This paper addresses the challenge of how such polycentric development can be achieved in planning practice. We introduce 'agglomeration potential maps' that visualize potential locations in a polycentric metropolitan area where positive agglomeration externalities can be optimized. These maps are utilized in the process of developing a new spatial vision for Flanders' polycentric 'metropolitan core area', commonly known as the Flemish Diamond. The spatial vision aspires to determine where predicted future population growth in the metropolitan core area could best be located, while both optimizing positive agglomeration externalities and maintaining its small-scale morphological character. Based on a literature review of optimum urban-size thresholds and our agglomeration potential maps, we document how such maps contributed to developing this spatial vision for the Flemish metropolitan core area.