Currently the definition of what is a battlefield is little different than "wherever we decide to put a drone". However, this leaves a lot to be desired from a rules-of-war and civil liberties perspective. The thought that merely visiting some city in the developing world and hanging around with the wrong folks could result in you dying in a battlefield via remote drone attack is more than a little disturbing, especially when compared against all of the normal heuristics a sensible human with good-intentions would typically use to avoid being involved in a war and especially being killed in battle.
Certainly there are risks and downsides to operating under rules-of-war which are too limiting or too strict but the solution is not to throw everything out and create effectively unlimited power with almost no safeguards, limitations, or accountability. And that is the road we seem to be heading down. As history has shown, power without accountability and limitations is never a good idea and will inevitably lead to intentional abuse.