Actually, I'd like to see a Heavy Autocannon that deals Direct Piercing and AoE Explosive.
What would make it balanced? Well, mid-range, hard to use, and it'd deal Light Weapon-levels of DPS. But it'd essentially be an all-in-one weapon.
It'd be kinda useful with other explosive weapons, and kinda useful with other piercing weapons. The trick would be making it fun to play with; it could be balanced number wise (just a matter of tweaking) but it'd need to be made interesting somehow.