Personally, I think the best change in regards to difficulty should come from the top up. A new trial of some kind at the end game to really test people's mettle would go a long way. It would give people something to do with those incredibly powerful builds they've thrown together.
This would, of course, be a lot of work. I'm probably underestimating the amount of effort that would go into such an idea. However, posthumous content additions such as Market Crash and the Vigilante/Rogue story arcs have actually been really well-polished and fun. If the team were to ever create a new challenge for players to test themselves against, I for one would think that it'd be exactly what the game needs.