The hook is the gameplay. But it is the gameplay at high player populations. The tactics of a low population server will be bland and repetitive because the tactical permutations are too limited.
If anything, Battlescape should have one or two servers, bringing the available player population into contact with each other other, just as EVE Online did.. If the game is a hit, then add more servers, but never so many that players don't find themselves surrounded by hundreds of others.
The game will be playable as a straight arena game with just a few players, but the real appeal is in the tactical spaceship combat that comes into its own when there are hundreds of players online. That's the marketing hook.
My ground truth here is the ARMA 3 server I support. At low player populations, sure you can HALO into the AO by yourself and shoot enemy infantry. But when you've got a solid player population, you've got helicopters transporting you out with others to join your squad in the field. As you make that trip, there are enemy aircraft being fended off by friendly combat air patrol. When you get to the AO, you face armor, infantry, and mortars. You and your teammates are working together to clear buildings and work across open areas. You probably have guys in your squad that can take out the light armor without needing to call support. But when you bump into problems bigger than your squad, you can call in support units to deal with them. When you go down, there will be a medic who comes to your aid. When you run out of ammunition, you can call on support to fly in a crate of ammunition. You can work with others to run combat support or logistics. You can be a pilot. There can be base air traffic control, ground JTAC calling in support, armor working together - all sorts of scenarios are played out. In general, there are many more moving parts, and the dynamics of the engagements are vastly more interesting.
That's the hook for that server. It's the thing that you throw in people's faces to say "This is what can happen on this server." And it does, day after day, because people want that dynamic tactical experience.
That's for a 60 player cooperative server. If PvP, it would devolve to a 30v30 experience, which just wouldn't have the same dynamics. So PvP has an even harder time getting into the stratosphere of tactical experiences.
I'll offer that ARMA 3 has lots of servers with lots of different game modes courtesy of its modding features. If Battlescape doesn't support modding, just skip the private server stuff. It will only dilute the population of the official servers.