End of the Arkhona story: The winning faction opens the ancient ark, spirits come out, faces melt, THE END

You forgot about this ;

Its a giant fucking ork that has lived since the beginning of their introduction into the war between the Slann and the Necrontyr.

That's certainly along the lines of what I'm thinking. I'm putting my money on one immensely potent artifact.

its always something huge and "devastating" the warp fragment in space marine, the blackstone fortress etc etc. Im sure whatever it is....it wont advance 40k's storyline

Hiding place for the lost legions of the first founding maybe?

I doubt arkhona will be a hinging point of the 40k millenuim, as there are more planets than arkhona/ Im finding difficulty in believing "whoever controls arkhona will control the galaxy".

Next thing you know, Makers will appear out of the ground and the factions will be fighting over a sacred spice...