I must go with Gregor here.
Either we condense the weapons or we give stuff from all the codexes.
And the proposed Heavy 0-6 6-24 - / -1 S9 2D6 -6 4+ High Impact 180TG is just a 80 tg cheaper version (because you have to buy a sight) of the multi melta with 1 str less.
I think the major issue is that some of those heavy weapons with 1 very powerfull shot are slightly overcosted since anything above str 7 gives little to no additional gain but does significantly increase the price.
Against your average ganger the heavy bolter (I know it goes out of ammo easier) has more chance to down/OoA him then an autocannon or melta, against anything with t4 the difference is minimal (slightly higher chance to OoA but less chance to pin), and against t5 the autocannon becomes a tiny bit better, the most expensive one (the multi melta) is equal (but with less chance to pin/down and no option to target multiple people) till t7 or an armour save of 1+!! The only possibility for that is an ogryn big muty with a thoughness increase... Which means that the autocannon with bigger range and cheaper is in every situation you will likely meed a better option. Early on the ammo rolls might change it, but by then the high t high wound people have also not yet appeared.
Those very powerfull weapons need something else (or a price decrease which I vote for), and I think that is why people want the cheap laser, because even with some drawbacks it might be worth it but the very expensive high str but only 1 shot weapons are so rarely worth it. People might take it because it looks cool and makes people a bit scared which can be nice, but usually a heavy bolter and similar makes people scared enough, the ingame effect of those heavy weapons is very little.