I'm not sure if this has been truly addressed elsewhere but I think games that have offered other objectives other than just "attack the enemy" and "defend your base" have always been more exciting to me. Having something like an aura tower that needs to be built up on the map. It may be towers that gain access to a new assault path, something that grants a specific powerful unit/siege weapon unpurchasable else where, or something that grants those around it a nice boost.
Also I'm not sure how much of a role "fog of war" will be playing, especially in the MMORTS, but the ability to manipulate "fog of war" and good monitoring of your enemy and their forces production has been key to becoming a true expert in any RTS I've played. Having alternative objectives that help you spy on your enemies hidden forces will be a huge draw for me.