Are we talking "improved" like improved Heavy Gauss Rifle or improved Heavy Laser, or improved like "what the HAG should have been?"
In either case, perhaps give them variable damage like RACs, but they can have either their current stats, or half their rating in one chunk. And then double the ammunition but make the current mode take two shots. The HAG-20 becomes a nifty alternative to the AC/10 family, the HAG-30 becomes a Gauss rifle that pays a ton for the ability to sandblast, and the HAG-40 becomes a complete menace to all that is good and right in the world. Mechs like the Onager that use HAGs as primary weapons gain a bit of damage concentration, the Clans get a unique weapon system, and the HAG is no longer almost totally inferior to Clan LRMs.