Even though there will not be playable elites in multiplayer I think that Invasion should still be in in a spartan on spartan form. Maybe in a different fashion than before but maps such as spire and boneyard were put into reach so that they could be used for invasion. If they are creating maps for invasion before I doubt that they would take it out of the next game if it was that important before.
Besides invasion is just a forgers' best friend. I loved creating my invasion map.