That's right. Appealing to multiple demographics is important because I:B has features that actually do. That needs to be expressed in the marketing. There's the true scale, seamless solar system for fans of exploration or the tech itself, also available in a sandbox mode at launch. There's the large potential capacity and cooperative gameplay, the Newtonian physics, unique take on the arena shooter, large-scale tactics, potential modding... Games used to truly start their marketing for release, but look at how many games are popular in beta. Marketing the beta would mean the release could deliver large concurrent player counts.
There are many game mechanics that could funnel players into the same place and time to ensure large battles. The locational battlescapes are a good idea, but what about 'temporal' battlescapes? Maybe not in a sense as strict as that. Perhaps small squads would either be player-formed outside of matches or selected randomly for those not in squads. A team would consist of a large number of squads. A squadleader, either voted in or by lot, could announce 'raids' while in a match. This would be a beacon or some UI feature visible to all members of that team, with squadleaders able to enlist their squad. The decision making process would just rely on voice or text comms, and then the 'raid' would display its date, time, place, and battle plan. This allows for lots of flexibility with multiple raids coordinating with each other and eliminates the problem of just a few players controlling the whole team. Also, it works even if there aren't massive battles. This would probably work best if all players in a match were in close timezones.
To prevent squads from all doing their own thing, just make certain tasks require more firepower, and these are necessary to win. The toughest resource base needs a team effort, same with the toughest station. Making the sites differ in difficulty (with more easier ones and fewer tough ones) would bring a natural progression from small skirmishes and few resources to fleet behavior and cap ships.
In an actual battle, since many players aren't used to voice comms and text comms are inconvenient in an action game, a signaling system with beacons/icons could be used for essential information.
The only thing I'm worried about is lack of replayability. Sure, the map is huge, but if it's the same each time, tactics are eventually going to get stale. Perhaps you could give each faction a subtle specialty, like cheaper capital ships. Maybe change the allegiances of the bases every game.