If you want true exotics, dig up the REALLY niche weapons:
- Thumper / Sniper / Longtom cannons (not artillery): Can even be house ruled to use artillery ammunition, despite being direct fire: Completely different firing mechanics (target the hex/ground, not the target, but in direct LOS)
- Centurion Weapon Systems: So obscure they can't even be found easily on Sarna, they are none the less capable of shutting down 'mechs and even DROPSHIPS. Just make sure to bring some heat-causing weapons and a lot of them, they aren't easy to knock someone out (but once you do you can keep them that way indefinitely).
- Silver Bullet Gauss Rifles: It's an LBX/15 - it's a Gauss Rifle - it's the Silver Bullet Gauss Rifle!
- 'mech bolos: Always worth looking into when you want odd, exotic weapons.
- The Blue Shield: Another oddball tech lost to the pages of history, the Blue Shield causes a huge lightning storm to emerge around your 'mech, and then it takes -half- damage from all PPC attacks. It's got a bunch of rules about operation, of course.
... and of course, there's always pretty much everything in Unbound, from VR head displays to torso mounted cockpits. I'd also recommend Solaris: The Reaches if you're looking for a huge list of potential really screwball modifications. Those are both set many years before your campaign, but none of the techs ever caught on, and would make you stand out really odd.
Most of this stuff (sans CWS, that's from an Interstellar Players book) can be found on sarna.net, with references to the books they're from. But I bet money most players never even have heard of this gear, let alone made it apart of their campaign.