The core of the second generation Topic Maps standards (TMDM, XTM2.0) has been finalized, yet the uptake is still slow. In this paper, we highlight engineering considerations for a novel backend for the TM4J open source topic maps engine, which is currently in development, but already usable for some purposes. As the name suggests, the “TMDM” backend is designed to reflect the TMDM specification closely. In fact, it is much closer to the TMDM than to the internal legacy TM4J data model (which is based on the XTM 1.0 data model). This motivates a bridging layer between the TMDM and the XTM 1.0 data model. We emphasize how merging is implemented in the “TMDM” backend and conclude with some synthetic merging benchmarks of the current “TMDM” backend prototype.