what i look for in multiplayer games is something that will keep me coming back for more.
if its too easy to get camped or its really easy to kill/get killed by people, the game loses interest.
if your making a fighting game, try including a wide variety of weapons and power ups, such as melee ranged and explosive weaponary, and health, temp stat increases, and other items.
if including classes/skills, it may be effective to make them strong against one thing, but weak against another to balance it out, such as characters with high power have low speed and vice versa,
i know this may not be much, but i hope these suggestions help you with coming up with ideas for your PvP game