I don't know if it make sense (I don't actually play the wargame), but maybe a Napalm grenade that serves as area denial? Like you throw it and the area covered by the explosion is on fire. You can pass through but take damage from it.
And units with las weapons and sat-lock can spec fire with them by bouncing the beam off a satellite.
Suppressors, i'd love those to be a thing, kinda weird to have all those special forces, but everyone go loud all the time except ninjas and stuff.
Maybe. The game already has Sepistor, which should be right terrifying to battlefield soldiers. Infinity does already have a morale like mechanic with Guts rolls. Rules wise, I am not sure how you would take the concept much further.
I think it can be safely assumed most weapons are some varying level of suppressed, or at least that the operation zone wouldn't warrant it. My issue with having firearms with Silent is that suppressors don't make your weapon silent, and any suppressors already present could be explained via the -3 to turn around. Like if you take an Infinity board to be roughly 300m by 300m (which could be off, but it feels right), suppressors aren't going to do much other than maybe mask your exact location, not hide the fact shots were fired to people on the board. Balance wise, I've never really been impressed with Silent even if you use it with stealthy CC stars like Shinobu, Oniwaban, etc. They were likely going to kill their target in hand to hand anyways, and hopefully you haven't deployed in such a way they can repeat that 2 to 3 times at little cost. And bows are just never a good idea.
I don't think what people mean by "terror weapons" (especially in futuristic setting) works through target's rational thinking. Otherwise any bullet would be considered a terror weapon. Monofilament templates from older editions were basically like this.
Instead of silent weapons, what if we go the other way with REALLY F@#*ING LOUD weapons? By which I mean, something like a "distraction" grenade (SQUAWKER) that forces affected units to change their facing? Spec fire, targetless, any unit in the circular template has to pass a WIP-3 check or turn around to face the center of the template. By itself, harmless enough. But now poor Atalanta is looking the wrong way and can't see the Krakot Renegade charging up her left flank.
In realistic terms, "silenced" is indeed a bit of a misnomer, so maybe a "suppressed" trait could happen: Silent, unless in their ZoC. Drawback, of course, is that's even more situational and rare to make use of than Silent unless your foe has made mistakes galore. OTOH it's the future and even a modern subsonic round with a good suppressor system can be pretty quiet, so who knows what ammo almost two centuries from now shot out of a caseless rifle can do? Also, I think 300m is a bit too much. If a mini is meant to be 1.8 meters tall, say, and is actually about 35mm excluding base it might be as small as 63 meters (48 inches is 1219mm, divide by 35 for 34.8, times that by 1.8 and round it up to nearest whole meter) but I'm bad at maths so...