Are there any others?
There's also built-in plugins but obviously those will really only work with OpenMPT, and there is absolutely zero guarantee for API stability for internal plugins (we refactor it as required).
DirectX plugins haven't really been relevant in the last ten years or so, I wouldn't even know where to find the SDK for developing them, and few hosts support them. VST2 is the only widely supported solution, and even though Steinberg is trying to make it more and more difficult to obtain access to the SDK (because they want to push VST3), there are third-party projects such as
DPF that will allow you to still create VST2 plugins. In fact, that project in particular allows you to use the same code base to create plugins in several formats. I have never used it myself but looking at the examples, DPF should be able to get you started quickly. The biggest limitation with any plugin format is that OpenMPT currently only supports one stereo input and one stereo output for each plugin (no sidehcain buses etc.), but apart from that you should be able to use most features of VST2.