In transport networks, each generation of optical line interfaces brings the prospect of even higher capacity and further reductions in the cost per bit transported. Currently deployed line interface technology already exploits coherent detection and advanced modulation formats to attain high spectral efficiency as well as to flexibly adapt the modulation format to the path characteristics, effectively trading-off capacity for reach. The upcoming generation of optical line interfaces will take these capabilities one step further by enriching the set of modulation formats supported and by doubling the maximum symbol rate at which each line interface can operate. This work presents a preliminary assessment of the network-wide benefits of deploying next-generation line interfaces. Considering two reference transport networks and modelling three different sets of channel formats, which approximately match three consecutive generations of line interface technology, a full design of the network is reported. The network simulation results highlight the capacity increase and the line interface and tributary card savings enabled by the upcoming generation of line interfaces.