Today during my lunch break I went down a weird rabbit hole of watching XCOM 2: Terror from the Deep play throughs on youtube. Perhaps it is just the use of the word 'terror' that forced a connection between these two things in my mind. But I got to thinking about the following high concept for a campaign:
Aliens have taken over the world, and the PCs are freedom fighters in underwater submarine bases engaged in a futile campaign of disruption and defiant violence against them.
The problem with turning a game like XCOM or Terror from the Deep into an RPG is that it falls prey to the old samurai sandbox problem. Having the players act like policemen or alien hunters forces them into a reactive mode which is repetitive and deprives them of agency. A campaign in which the PCs are elite soldiers or superheroes staving off alien attack is one in which they are basically at the beck and call of the DM, either directly because they are generally carrying out missions on behalf of some higher authority, or indirectly because their job is to wait for something bad to happen and then ride to the rescue like cavalry. You can possibly have a fun game that way, of course, but it won't be a sandbox and the players won't have much choice about what to do. It'll be more like a wargame with acting. It'll also be hard on the DM, who has to constantly come up with interesting scenarios rather than following the players' lead, which is one of the main virtues of a traditional sandbox campaign.
(This is the same problem in a nutshell as a "the PCs are cops" campaign: following orders and waiting for bad stuff to happen before acting is not as interesting as its opposite.)
But spin Terror from the Deep on its head and you have something very different. The basic idea behind TftD, for those who haven't played it, is that aliens who have been slumbering on the ocean floor for millennia have suddenly been awakened and are rising up from the oceans to attack human cities, spreading fear, panic, destruction, blah blah, and a special international agency has been set up to stop them. The parallel universe version is that the aliens have taken over the world, and the humans, rather than stopping alien terror, are engaged in a terrorist campaign against the alien overlords.
Rather than casting the PCs as guardians of humanity, in a reactive/defensive role, it makes them purposive actors - planning activities (bombs, raids, assassinations, whatever) - with the whole world as their oyster. Each session they're not looking to the DM to discover what their mission is; they're deciding for themselves how they are going to further their terrorist campaign.
A concluding observation is that players love this sort of thing. I find that the best moments in RPGs, the times when things really seem to sing the most, come when the PCs are coming up with some dastardly plot or scheme; that's when the players come to inhabit their PCs the most, and the point at which distinction between player and character begins to break down. When players are cooking up a plan, they start to truly think 'in character', and they do it collectively into the bargain. A terrorist campaign is one which maximises the opportunities for precisely that kind of mood to develop.