SDL and OpenGL are a safe bet for AmigaOS 4.x and MorphOS. Just remember to use the "SDL/endian.h" include when accessing the buffers of any files that have been loaded in so that it can correct the endianness of your PC code.
Just don't use display lists or shaders in your OpenGL code because the MiniGL implementation used by OS 4 doesn't support them.
As for the high-level languages, you may have to wait a while becuase Mattathias Basic hasn't gotten a working object-oriented extension format yet and Python for AmigaOS 4 is buggy and doesn't have the PyGame extension (SDL for Python). C++ is an option though.
