I completely agree about the "magnitude" of work required to make a worthy a-la-SC2 game. Its sure no easy work. But i see the difficult part only in the story, the GFX itself shouldnt be that much of a problem.
* Enlarging the existing SC2 space is i think not such a big deal, all is allready there, the planet types, the metals, the biocrap, the energy elements, adding new would be a piece of cake if you ask me.
* Creating new "aliens" is a matter of equilibrium between the ships in the arcade aspect, but only difficult part is to make the new ships and create the "outlook" of the races, to look comparable to existing SC2 races and add the apropriate dialogues and music.
* Adding a bunch of quests and missions should also be relatively easy, since the game engine is allready there.
* As for the story, SC2 has more then enough clues to make a new one... like bringing back the old Sentient Mileu races, the fallen Burvixti (sorry for the spelling), the SC3 story - secret of Precursors - made anew, the Pkunk/Yehat story, the Ur-Quan comeback., the secret of Mark IV ship, the Ur-Quan finding Dynnhari (spelling again). Or look at Slylandro, Melnorme and other "history" reports, there is more then enough material to work with. <Or just take a good look in the Kohr-Ah trophy pit, more then enough material there>.
* Not to mention all other possible aspects like mining, refitting the ships, developing tech even etc... that could be added.
If there would be such a project i would be more then ready to take part in it, cause i find SC2 space a true pearl.
Yours,
J.W.