The demonstration of organic bipolar transistor in 2022 completed the missing puzzle in the organic transistor development roadmap. The major obstacle was that the typical amorphous organic semiconductors could not offer high enough carrier mobility to allow sufficiently long minority carrier diffusion length, which is central to bipolar transistor operation. The use of epitaxially grown rubrene with doping together with a novel vertical transistor architecture helps to overcome the carrier mobility challenge and lays the foundation for operational bipolar transistor. Bipolar transistors based on crystalline rubrene exhibit high-speed operation in the gigahertz range and allow direct measurement of minority carrier diffusion. In this perspective, we discuss important aspects in terms of device design and materials for further advances in organic bipolar transistor device performance. We also point out emerging device concepts, such as organic heterojunction bipolar transistor and phototransistor, that could stem out from organic bipolar transistor development and cover further work necessary.