I want this game mode so bad man. Why can't R* give us what must surely be very simple to program into the game?
Would you swap all missions and DMs for it? I know I would.
Just think of the possibilities on GTA V vs IV.
Extra spawn locations for Getaway. Lobbies where your rank matches that of other players in the game, so no crap bosses. Option to turn Helis on or off. Penalty for Cops who destroy the Getaway. Option to wear different Cop uniforms.
Keep it simple though!