Ultramarines are going to be in the next box set, it is just the way it is going to be. We all know that, so no need to further expand on that idea. The terminator and dreadnought models make the kit worth the buy as it is and the 10 standard marines and the commander is just icing on the cake for marine fans. I would throw in a rhino or a drop pod just to really sweeten the deal though...
Tau would be a good second set to clash against the smurfs. I like that idea. They do have a draw on the younger crowd with the anime connection. I personally do not like the tau models, but that is just my personal tastes.
Nids would also be a good choice even though they were done before. Enough time has passed to throw nids in again, especially with all the new models available. That would be a contender for sure.
CSM, not going to happen. People new to the system would be slightly confused as to which marines were who and who belonged to what. I just don't see it. That and it would reduce the model count in the box by too much. Face it, the high number of orks in the current box really makes it stand out as a great deal for the new and seasoned consumer.
Any other imperial force isn't going to be in a new box, they can't clash with the Ultramarines, doesn't make sense to a new player or even the fluff that is current. They also are not going to drop marines from the box, so they are out all together. The only slight possibility would be guard instead of marines, but that is such a long shot it is but a pipe dream.
Necrons would be cool, no doubt about it. Very feasible and interesting for the box to have them included. A good choice. With new stuff reportedly on the way, it would be a reasonable choice to include them. I for one would buy the set if it were necrons as I have always wanted to do an army.
Dark Eldar would be another decent choice, but that will depend on the new codex and rules put out for them. As they stand, that army is more for an experienced gamer so that is a drawback for including them. Again, model count would also be a factor for not having them, not enough in the box to make it a great deal.
Eldar, maybe, but again, just like the Dark Eldar, not the easiest army for a new person to jump into 40k with and model count comes into play here too.
Orks have been done, but that doesn't mean we might not see them again. Drop the deff copters and put in a trukk or bikers or even a deff dread or dare I say, some killa kans, and the box set would be fresh as a daisy. Maybe even buff up the nobs to a 10 count to make it a super sexy deal.
My vote would be nids or necrons, either one would make for a decent boxed set that could be expanded quite easily. Second choice would be Orks again, albeit with a better support choice over the deff copters and more nobs.
The one thing I would really love to see in a boxed set would be complete codices for both armies, (which ever ones they end up being) included. I think that would be a huge draw to the experienced gamer to buy what is considered an introduction set if it came with very complete books. They would draw sales from new and old alike if they did that. Even if they were miniaturized like the AoBR BRB, it would still be a major draw.