One of the things I enjoyed most about C&C Generals was its single player replay-ablility. I loved spending hours navigating through the General's challenges or custom maps, and coming up with unique (open-ended) ways to accomplish the mission the way I desire. Re-focusing on this aspect of the game and creating an online, massive, constantly updated, designer/user created, and user available library of challenge maps would be a great addition to the next C&C game.
The first goal of the massive challange map library, would be compelling challenge map design (greatly expanding on the Generals: Zero Hour/Uprising design); that sticks to the Generals/C&C3 gameplay (re-focusing on base/defense building and larger unit counts). The maps should be massive to give the user a lot of room to be creative and expand. The map challenges should have varying levels of difficulty, with some specifically for singer-player, and some specifically for co-op (so as not to force single players to have to run with an AI ally for ALL the maps). The focus here is replay-ability, and user creativity vs an AI opponent, so linear story driven missions such as the ones in C&C4, SC2, Red Alert 3, World in Conflict, and Company of Heroes, should be avoided. There definitly needs to be an achievement system based on certain styles of gameplay, to compel the player to replay the map with different units than before (armor, air, infantry, etc), or what they are use to with different tactics. The key here is open-ended gameplay in a RTS (think Battlefield or GTA concept, but for an RTS); not linear "train track" like gameplay. The library for these challenge maps should be huge! Since there is no real need to stick with a storyline, or pvp balance; the sky is the limit with the amount of creativity the designers and/or users can come up with focusing on player vs. AI.
This idea would cater to the casual player; which I believe most RTS fans tend to be. I personally know a lot of people (and I am sure there are more who would agree) who play through single player campaigns and then call it quits (C&C4, SC2, Red Alert 3, World in Conflict, and Company of Heroes for me). This Challenge Library would entice gamers to stick with the game, play it for years, and tell their friends to check it out (like what I did with Generals).
There are some who would disagree with me, and say that online player vs. player is the most important aspect in which you have to design an RTS game around. If this is true statistically; then this would be a bad idea and many people wouldn't care, or contribute to it.
Bring it Back! Make it Online! And Make it MASSIVE!!