From a game design perspective, implementing PvE out of the blue has the potential to wreak unprecedented havoc in an exclusively PvP game.
I personally would love a properly-done implementation of PvE.
In APB, however, I don't see PvE being a safe direction to include into the game, with essentially all of its design being centered around PvP.
This, and besides all the technical and resource challenges, PvE could very easily damage the value of PvP gameplay for everyone except the most skilled competitive players. PvE could bring in more players, but, of those who would continue to play, it would divide the existing population and their time between PvP and PvE. This is problematic because PvP players will have their matchmaking pool reduced, furthering one of the problems we face with today's matchmaking.
From this perspective, I'd only consider entertaining the thought of PvE after matchmaking is improved.
