We have published a new paper—published as open access—explaining why we have developed CityJSON, and it also presents the main engineering decisions we took:
@Article{19_ogdss_cityjson,
author = {Hugo Ledoux and Ken Arroyo Ohori and Kavisha Kumar and Bal{\'{a}}zs Dukai and Anna Labetski and Stelios Vitalis},
title = { {CityJSON}: a compact and easy-to-use encoding of the {CityGML} data model},
journal = {Open Geospatial Data, Software and Standards},
year = {2019},
volume = {4},
number = {4},
doi = {http://dx.doi.org/10.1186/s40965-019-0064-0},
}