Well, here are a couple things that I'll repeat, and suggest below. These are the things that I believe could greatly improve PvP on the server.
1. A necessity for PvP would, obviously, make factions want to PvP more. A few suggestions here could be to reduce money from monsters, and increase money from players. This would make players wanting to purchase things need to kill players for a faster income. Not sure about this one. The next one is that we can enable some way to get into faction bases. This will convince many players to come out of their bases and raid, and for others to attempt to build defenses to stop them. Finally, creating a more hostile enviornment outside of our spawn would lead to more PvP, out of necessity to thrive. This suggestion is not one I'd recommend above others though, as it's complicated and would require a map reset. Here's what it is: We can get a plugin developer to create a simple generation code (I could probably do this) that will cause coal, iron, gold, redstone, etc, to spawn in smaller veins, and a lot less. With this, players would need to PvP and raid to gather more resources.
2. Capturable areas (not battlegrounds, you'd lose your things when you die, not items generated when you go to the area) with benefits, such as cheap shops or healing areas. If we could motivate factions to capture and fight over these, we'd see a lot of PvP. Of course, this would require somebody who really knows how to code plugins to make it. It would also need some way of finding out what factions there are and who are in those factions.
That's all I've got for now, I'll edit this if I think of anything else.