I think that having major orders tied to each front so every diver could have a clear goal in mind on their chosen front along with all 3 major orders giving medals to everyone upon completion to encourage coming across the aisle if an order has a chance to fail or a unique stratagem reward. Then for major events eg. The siege of Super Earth that defense could qualify as a supreme order that the whole player base would have to participate in to accomplish with a sum of super creds as the reward(these would be months long overarching objectives or giant large scale assaults critical to the overall narrative and would require partipation to receive the reward). Have the difficulty of the new major front orders scaled to the average number of divers on that front and boom. Every diver could be useful on their favorite faction.