The ONLY thing I could really want extra from OpenGL at this point is an uncapped framerate, or at least a higher cap, perhaps with some kind of linear interpolation of the various movement going on to fill in the resulting extra frames. It'd be pretty.
Of course, I'm pretty sure that subject has come up at LEAST once, and then it was stated that it wasn't gonna happen. But hell, I can dream.
That being said, if SRB2 were to hop gears to any other source port - and it won't, ever - then my vote goes to GZDOOM, plain and simple. Slopes AND FOFs, Dynamic lighting, ACS (as opposed to linedef executors), DECORATE, and tons more; aside from the utter hell that coding such a modification would produce, what's not to love?