Honestly, I don't think Heavy Fighters need anything particularly fancy done to them. As some have said on this thread, size is basically one of the most important factors on what makes a ship good in this game.
HF's should be what we see now in certain HF's like the Black Dragon or other kusari HF's, minus the fact that they tend to do more or less the same damage as VHF's. Restricting them to class 7 guns really isn't a downside and tbh I don't know why it was slapped in there. It made sense with the previous "class 7's do less damage and are more energy efficient!" but now it's just kind of silly. Reduce the core on HF's and give them the choice of either filling their ship with guns and not being able to shoot as often or being more conservative on how many miniguns they take onto the school bus in return for more shots. Simpler than making a whole new tier of guns. Oh, and remove the stupid class 8 restriction. Serves no purpose. Hell, let them use codenames. Not like codenames are that special unless a selling point is if your weapon has a name in all capital letters.
Let's not kid ourselves about LF's though. I haven't seen one used for intercepting since a year after I first joined Disco. It's a whole ship class designed for a task that nobody really uses it for. Disco = PVP first and foremost. Unless it's good in a scrap, people likely won't use it. I used to like them but missiles make flying them a major pain. I don't really see enough pay-off for their fragile state. On one side, I'd support buffing their damage and removing their CM so missiles hard counter but I don't think many people want to fly ships that are just automatically killed by something else. so basically I got nothing for LF's.