The rapid development and continuous updating of the mega satellite constellation (MSC)have brought new visions for the future 6G cover-age extension,where the global seamless signal cov-erage can realize ubiquitous services for user termi-nals.However,global traffic demands present non-uniform characteristics.Therefore,how to ensure the on-demand service coverage for the specific traffic de-mand,i.e.,the ratio of traffic density to service re-quirement per unit area,is the core issue of 6G wire-less coverage extension exploiting the MSC.To this regard,this paper first discusses the open challenges to reveal the future direction of 6G wireless coverage extension from the perspective of key factors affecting service coverage performance,i.e.,the network access capacity,space segment capacity and their matching-relationship.Furthermore,we elaborate on the key factors affecting effective matchings of the aforemen-tioned aspects,thereby improving service coverage ca-pability.