The Demolisher's Micro Rockets and the Lancer's Particle Lance were inspired by or even reverse engineered from Yautja Plasmacasters
Although obviously left out of Fireteam Elite, it's more than likely the Colonial Marines have had encounters with the Yautja and observations of the Plasmacaster in action most likely left an impression on the upper ranks.A hands-free shoulder-mounted and computer targeted heavy weapon system obviously has much appeal for force multiplication so research and development to produce human equivalents was initiated, eventually resulting in the Micro Rockets and Particle Lance. It may even be possible that a few working Plasmacasters were recovered and studied in the development.
The Yautja origins are clearly seen with the Micro Rockets using a targeting laser prior to firing and the Particle Lance basically being the human equivalent to the Plasmacaster.
