While you are right that SM4 features were really well covered by nVidia at the launch of their G80 chips, i would like to point out that even today ATI does only expose a fraction of those features in OpenGL.
Putting all the SM5 stuff into one big extension might force ATI to support it completely. But maybe before one takes the next step, it would be better to finish the current step. Put the current SM4 stuff into core, maybe even try it with that über-extension idea. Just make sure, that we get sensible SM4 support on ALL hardware, before you concern yourselves with SM5.
To be honest, i can’t give you a top-5 list, since all that i heard about SM5 sounded useless / not important to me. I am not even using all the SM4 stuff, some of it is just too specific for certain problems (which i don’t have right now). SM5 while nice to have, of course, is not of much interest for me, at all.