According to the official website, an OVA of Itsuka Tenma no Kuro Usagi
was announced to be bundled with the new volume of the novel. The original author Kagami Takaya
will write a new episode based on fans' tweets. You can make a request about the episode via twitter with #itsutenanitsuku hash tag.
Source: Itsu Ten official website
Itsuka Tenma no Kuro Usagi OVA