As the new engine update is coming soon it is expected to have a rise in player base / popularity of the game. My suggestion would be adding more selections in music and symbol customization and do some rework.
Symbol:
New basic symbols with different curves. More symbols especially for hair are needed. Also increase the amount of symbols for premium accounts. I understand that APB has a function of "complexity' and it cannot have infinite amount of symbols placed in one picture but this can be achieved by installing some new features which could merge dozens of symbols into one. By that I mean increasing maximum amount of symbols and automatically utilizing the whole work after finishing to fit into the frames of so called 'complexity'.
Music: increase all kill-themes duration to 10 seconds. Most themes are based on real songs and in order to fit a piece into 5 seconds duration people have to either increase the tempo or cut out some parts. There could be really good works after this is implemented. Also add some more sound kits to music studio or make their customization more complex.
In summary I'd like to suggest to increase the complexity and utilization of music studio and image creating along with adding more features and stuff.