Other possible things that could be done with Plasma Carronades to give them a niche would be say, something like giving them the ability to do damage like missiles when fired from orbital support, since they are also large-blast, indiscriminate weapons.
Personally, I'd also love a tech that lets you mount carronades that are of an older generation than your current tech (say one-half your current caliber and lower, maybe two-thirds?) on dedicated PD turrets that would get a pretty hefty to-hit bonus because, again, it's a wide blast, indiscriminate weapon. Maybe one that is tuned to be better at area defense than final protective fire, so you have, effectively, ranged energy weapon flak.
Of course, for something like this to work well, we'd need formations back, and as far as I can tell, those are AWOL without any posts from Steve on the topic, at least any that are collected up with the rest.