Network controller

Hello Networker, Wanted to get a general feedback, if you are considering having a controller on your network. Let's say it will be on your core network to handle your mpls network or core network or to provide Traffic Engineering (TE) feedback/decisions What would you need from a controller and its software What we would like to have in a controller and its software what we can live without it ( The answer isnot the controller here) Cdt LG