The enrolment of third-party developers is essential to leverage the creation and evolution of data ecosystems. When such complementary development takes place without any organizational consent, however, it causes new social and technical problems to be solved. In this paper, we advance platform emulation as a theoretical perspective to explore the nature of such problem-solving in the realm of open platforms. Empirically, our analysis builds on a 10-year action design research effort together with a Swedish authority. Its deliberate change agenda was to transform unsolicited third-party development into a sanctioned data ecosystem, which led to a live open platform that is still in production use. Theoretically, we synthesize and extend received theory on open platforms and offer novel product and process principles for this class of digital platforms.