A preplanning framework to evaluate the potential of different modulation formats in dense wavelength division multiplexing (DWDM) networks is proposed. This framework consists in analyzing the number of feasible paths and transparent connectivity for each modulation format, enabling to shortlist the modulation format(s) more suitable for a given network, thereby potentially simplifying and speeding up the full planning process. In order to exemplify the usefulness of the preplanning framework, four different reference networks are considered. This study supports that 8- and 16-quadrature amplitude modulation (QAM) are suitable formats for metro/regional networks and that, on the other hand, quadrature phase-shift keying (QPSK) is still essential to mitigate costly 3R regeneration in long-haul networks. Moreover, the proposed framework is also exploited to evaluate the improvement achieved by using Raman amplification in each network, showing that it can be key to increase the usage of the more spectral efficient higher-order modulation formats.