The subject of what will happen in the actual Siege of Orgrimmar has been discussed many times over, but there is very little discussion on what the non-raid content of patch 5.4 will be.
Part of this is because there isn't a whole lot of information out there, but we do have something. In The Secrets of Ragefire when you examine a stockpile of weapons, your character speculates that Garrosh may be planning an invasion. Since the whole scenario is likely a preview for 5.4 environments, we can't really ignore any details given within.
Given that the rebellion and the Alliance are seemingly closing in on Orgrimmar in patch 5.3, it would seem odd that Garrosh is in much of a position to invade anywhere. Unless he is concealing his true capabilities to lure his enemies into making a tactical error...which wouldn't be the first time he's done exactly that.
I'm going to theorize that while we Siege Orgrimmar, Garrosh is going to be attacking Alliance and rebel-controlled Horde cities. Depending on how ambitious Blizzard is, potentially all racial capitals will be attacked in some form or another. This would probably demand a combination of scenarios and rotating daily hubs to cover every city, but it could be done. If not every city could be covered, Alliance cities should be prioritized given that the raid will take place in a Horde city.
Now of course there is the question of how Garrosh has the forces to launch so many attacks and defend Orgrimmar at the same time, but there are plenty of potential answers here. The attack on Stormwind might for example involve a handful of Dark Shaman summoning a bunch of bound water elementals and Kraken, while the attack on Thunder Bluff might utilize some sha-powered weapons.
The overall idea would be to push the impression that, if the Siege fails, the very existence of the Alliance and the Horde rebellion is in immediate peril.