MRIcroGL is a natively compiled executable using the native widgetset of your operating system (Windows API for Windows, QT or GTK2 for Linux and Cocoa for MacOS), with Harare accelerated graphics and compute using OpenGL (or optionally Metal for MacOS). Unlike FSLeyes, it is not written in Python, rather Python commands are interpreted (using PyArg_ParseTuple) as native commands. The benefit is that it is very fast, the disadvantage is that you can not run it directly from the Python command line. In this respect, it is like Blender, where you can launch it from the Python command line and have it load a Python script. If you want to handle NIfTI images from the Python command line, I would think FSLeyes would be a good choice. On the other hand, MRIcroGL is open source, so if you want to extend it, you could always add in hooks for IPC. If you want to develop this, create a fork of the Github repository, and when you are happy with your solution generate a pull request to share your solution with the community.