As someone who's been getting more into the slummer and rebel space on main lately, I don't have too much to add that others haven't already mentioned, but I'd like to word it in my own way and add some real world knowledge of how irl insurgencies and partisan movements function and how that could be translated into the game narratively and such.
An important thing mentioned by Gr4Ss is the fact that rebels have the advantage of being able to choose when to fight their battles. Historically, this is the big thing with how insurgent movements irl were able to make any kind of progress against whatever occupation force they were fighting against, and is generally considered to be one of the primary advantages that insurgencies have over the occupation forces in terms of how they fight. The quote typically goes "The guerrilla wins if he doesn't lose. The occupation forces lose if they don't win." Basically, what this means is that, while insurgents can choose their battles most of the time to ensure they take on operations with the highest chance of success while minimizing losses on their end, the occupation forces must always win every encounter they have with the insurgents to be making some kind of dent in their activities, hence why counter-insurgency campaigns have historically had "search and destroy" doctrines applied to them. There's a reason why insurgents doing a quick ambush then running away into no-go zones before the occupation forces can catch them is considered a loss for the occupation forces, even if the ambush itself isn't particularly successful, as the insurgents would be able to simply escape and live to fight another day.
So, this is why it takes a while for rebels to do much of anything in the city and how they prefer to do most of their activities on expeditions, because the city is far from an ideal location for insurgent activity to take place in due to how well-guarded it is and the generally tight grip Civil Protection keeps beyond the slums. Short of some terrorist attacks and HVT assassinations or a general uprising that may occur after potentially years of outland operations, planning and prep-work, rebel activity shouldn't realistically be expected to occur within city limits too often, as most of it would take place in the outlands, where the Combine presence is weaker as others have mentioned, and the rebels are able to use the rural and remote terrains to their advantage more.
One thing that, in my opinion, has proved that rebels need some kind of shared narrative to get behind in order to do their activities more, is the addition of the quest board in the slums. The quest board, which is typically operated by GMs offering work in exchange for chips, has allowed more rebel activity to occur within the city due to the nature of the work offered and the payments offered, and the fact that the quest board allows for rebels to do things for rewards more often than they would've under the old system of waiting for BMDs to give people quests from time to time (which they, of course, still do, but it works better under this system where they don't have a monopoly on quest giving). In my opinion, I think that rebel operations during expeditions should also have an effect on the Combine's resources and security situation within City 24, and that Civil Protection/OTA/OAA expeditions should be done to counter outland rebel activity and maintain the tight grip in City 24, basically serving as a foil to rebel expeditions whose success could potentially bring stability and security within the city down. Ideally, as the security situation within City 24 changes for better or for worse, quests given by GMs on the board will also change with the kinds of jobs they have to do and the rewards offered, in order to reflect the changing situation within the city at different times.
In my opinion, the primary goal that rebels should be working towards with these expeditions to bring stability down in the sector around City 24 (and potentially Sector 10 in general depending on how people would want this to go), would be to weaken the local Combine presence enough, as well as allow for enough resources to be funneled to the rebel side, to eventually allow enough equipment and fighters to be smuggled into the city to make way for a general uprising, ala the Warsaw Uprising of 1944. And by "general uprising" I don't just mean "five rebel cells show up in Triumph one day before getting assblasted by OTA" or whatever. An event for an uprising to occur should definitely have heavy GM involvement and such so as to better ensure that the rebels have an actual chance to succeed at doing this, and also for the narrative team and such to make it clear that it's a city-wide uprising and not just restricted to the district we're playing in, in order to provide better payoff for all the work rebels would've been doing in expeditions and such to make this event happen, while giving the highest risks for failure as well, since failure would mean the rebel cells being wiped out and forced to start from square one again, or even the city itself being wiped out by the Combine or at least the CCA and such being fully replaced by entirely new people.
Now, the pacing of rebel activities within the narrative to eventually lead to something like this can vary, but I think a good thing to use as a reference point is the doctrine of Protracted People's War and how insurgencies operate according to that doctrine. It's a three-phase doctrine, detailing a mixture of political/propaganda and military actions that guerrilla groups should take to fight occupation forces, from the recruiting/equipment gathering/building of a base area phase, to the guerrilla activity/terrorism/base area expansion and establishment of new base areas phase, to the final phase of conventional warfare, which would be the uprising/civil war phase in this case.
Now, I know that some narrative restrictions still need to take place due to the lore and timeline on Willard, but with v3 on it's way soon, I think having this kind of narrative potentially play out for rebels would be one good way that we can cap off v2 before moving to v3 in a way that still allows for player agency to help determine the ending of v2 and such. Even besides that and going into v3, I'm kind of into the idea of uprisings happening in places outside of City 17 before Gordon's arrival in the narrative anyway, with the Combine either destroying them before they spread beyond their home sector, or keeping them relatively under control in a stalemate or something while heavily censoring any info about them occuring so that most of the citizenry in sectors beyond where the uprising/civil war is happening remain largely unaware that it's even happening, and think that everything's under control.
At the end of the day though, quite a bit of this is just wishful thinking on my part and such, since I'm pretty sure staff already have a plan for how to handle rebels when it comes to expeditions and city activity and shit, but I figured I'd throw my 2 cents in here too, just to give people ideas.
