I'm gonna shy away from hard numbers (since we haven't seen hardly any SF weapons yet), but it would certainly be a heavy weapon (meaning it has a minimum Strength requirement), it would do bludgeoning damage (perhaps half as much as a comparably leveled weapon), and it would allow the wielder to make a free bull rush attempt once per round upon inflicting damage to a target.
From what I've seen, you can make this weapon a few different ways. I'm not too familiar with how Starfinder handles damage, so doing it Pathfinder style.
It could be a weapon that deals force damage anywhere from 1d8-2d6, not sure on the firing rate (probably functions as a normal weapon; not auto/semi).
Not 100% sure about how the knockback effect should be, as it is seen used on those who didn't display any competence/power beyond 1 npc level. It could be cmb with intelligence modifier on trip and knockback upon crit, and those basically defeated by it gets affected by it through flavor. Another method can be fort save (like anywhere from DC 11-15) and get pushed a square back and get tripped upon being damaged by it.
If it's done as a critical effect I'd wait until we see how critical chances have changed with auto-crits built in. If it's main shtick is the displacement then relegating that to a crit might make it too niche to bother with out a higher base payoff.
That'd put it closer to what Sauce is describing, though i think I read cmb/cmd might be on the way out alongside combat maneuvers altogether.
All weapons have the same crit range - 20/x2, so not great for a primary shtick.
CMB & CMD are out, but combat maneuvers are not; they're basically attack rolls against KAC with some sort of constant modifier.
I like the idea of a Fort. or Ref. save more than a maneuver, honestly. Easier to balance.
Make it be a maneuver and a save maybe.