I'd like to see a massive Diorama of maybe Ullanor, when the Emperor made Horus Warmaster. Have all the primarchs that were present at Ullanor there, with the Emp, hundreds of SM from all 20 (18) Chapters represented, lots and lots of IG units, Titans. The whole kit and kaboodle. But it would be strictly just a scenery/diorama thing.
I've seen some pretty massive projects that some GW stores have done, so I know it's possible. But for the most part pretty unlikely. The Primarchs should only be created once, and used just for scenery purposes. I just don't feel all that inspired by a lone Primarch standing there, it would have to be represented in a battle scene or something of great importance.
Just my 2 cents worth.