For me it's important to keep it a lot like the original game but more open world, more pvp, more end game content like doing big missions together. Also the architect place to make your own content should have a lot of (balanced) options.
The cities should feel immersive. Like there is a lot of random things going on with the people, police etc. and it should generally feel like a real city. People buying, selling, talking, laughing, media outlets with news about current events. Anything that makes you feel like you are in a real city. Also if you're a Hero you should get penalties for doing unlawful things and if you take it too far you become a villain. If you are a villain and you are doing good things you should become a hero. More fluid than going rogue (no rogue, if you go neutral you are a citizen. No superpowers but still able to do gta-like things). You can get caught by police or chased by gangs. Make sure the choices people make have real impact but it should not be easy to get on top. The work to really get good at what you do should be meaningful.
Working together should stay a big part of the game. Community vitalises the game. So cities can be taken over by villains and gang but can be reclaimed by the heroes.
Don't focus too much on making the game pretty. Meaningful gameplay and immersion is what is important. Don't make the DLC mistake. Let people reward you for making a good game with donations (people who love your game are generous and if they see you have a fair model compared to others i think you will get more rewards than the greedy cashcow tactics of today) or buying the full game and perhaps with a low monthly fee. But make sure the time you pay for only is spent in the game. Not when you are not playing it.
