I think the Catachans should stay around, though they should be revamped to look more like the current command squad set and less like 80's action figures. I also think GW should do the other Guardsmen flavors in new plastic sets (Steel Legion, Mordians, Tallarn, Vostroyans, etc.) It would add a lot of variety and spice to the infantry, and it would play up the fact that the Imperial Guard come from a diversity of planets all over the vast Imperium.