Publicly available CityGML datasets containing multiple levels of detail are scarce. For this reason Random3Dcity was built by Filip Biljecki at TU Delft. Random3DCity is a procedural modelling engine for generating synthetic buildings and their realisation in CityGML in multiple LODs. The generated datasets are used for multiple purposes within Filip's PhD project, but possible applications are not limited to it.
Filip has publicly released the datasets, see https://filipbiljecki.com/code/Random3Dcity.html