I'll start by stating an absolute: Avengers are the only viable weapon in PvP currently.
EDIT: Avengers and rocket launchers. My most insincere apologies to the single guy who absolutely had to whine about this to me even though that's not the point of this thread.
Expanding, viability of weapons is a matter of weighing usefulness to availability. Avengers are commonplace and perhaps one of the easiest Big Guns to get. They are currently the highest damaging weapon in the game all things considered, ergo they are the most prevalent in PvP. Balancing these is not necessarily an issue of their damage or immediate usefulness in PvP but also their availability. Consider how easy they are to find as loot from various sources and to craft, consider their range, damage, and how commonplace the ammunition is with even better conditions. Now it becomes obvious that all other weapons are discouraged from use in PvP, which leads to the act of fighting other players being incredibly uninteresting and devoid of diversity.
Specifically, the issue of weapon viability in PvP is also an issue of economy, where gross weapon availability nets the constant use of the best possible weapons: avenger miniguns. Fixing the economy will make great strides in increasing weapon diversity in PvP. Actually balancing all of the weapons is another way to go about it, which I'm definitely a supporter of. Currently weapons in general do far too much damage for any semblance of tactics to be viable. It's currently a matter of standing in a firing line and waiting in the case of defense, and in the case of offense locating said firing line and attacking it from the side. Everyone uses an Avenger and a rocket launcher or they aren't viable.
How these weapons are actually balanced appears to be entirely up to the developers, being aware that most the community is heavily biased. Although not the purpose of this thread, I'd like to recommend all BGs force walking. Big Gun users would become far more stationary and their role in PvP would be different, rather than making Small Gun users redundant, or Energy Weapon users worthless. It would encourage the use and effectiveness of ambushes, and even a wealth of availability of big guns would not cause them to overshadow all other weapons.