To be honest what constitute a modern city is considering several factor
1. Their habitants or population, a modern city always had high HDI among its population so usually they had potential in Financial economy might, manufacturing industry and creative industry to sustain their economy growth at long term and becoming a political and cultural center in the region.
2. Infrastructure, modern city usually got top notch infrastructure buildings to support the activities of their population. Not only at transportation, healthcare, housing, water treatment, administration, government etc even they got leisure and green infrastructure to sustain their population activities at healthy level
3. Connectivity level, modern cities usually got high level of connectivity among other important cities to support their continue growth. For example, Kuala Lumpur, Jakarta and Singapore had high connectivity among each other, not only at transportation issue, but even at people to people level of contact among population of those cities is very intense and close