There's nothing really stopping Tri-Tachyon from
being "that other superpower". Other than David, of course.
From the current backstory, it's pretty clear that corporations had enormous influence in the Domain. Such that it's not unreasonable to see a fraction of one emerge as a major power in the Sector. I could easily see a Cold War going on between the Tri-Tach and the Hegemony. Neither of them would be inclined to get into a real war, since that would cost resources they don't really have, but proxy wars and bullying the many smaller factions in the Sector is fair game.
The two of them falling into a Sector-wide war could be a major event that could randomly happen in the campaign, if forces happen to align to cause such a thing. I would prefer that the player not be able to instigate a full Sector War, at least not without lots of effort and subterfuge.
I kind of feel a Sector War would really be the end of the Sector. So much would be lost that the whole Sector, excluding a few terraformed worlds (that also weren't destroyed in said war), would essentially be depopulated by starvation. Unless, that is, a third faction is instigating the war, and succeeds in its bid to claim power and emerge as the only superpower in the sector before too much is destroyed. *Cough* the player *cough*. Then the whole sector is your playground to form a new order.
Of course, anything like that will only come into existence in the far future of this game's development. And I would hope there would be at least several other end-game goals besides bringing one faction to dominance.