I feel like it's important to keep in mind context and think of a way to implement content/trigger warnings while designing your rating system, because I feel it would be hard to implement after the fact and there will be some overlap. It's difficult because there are plenty of adults that are just fine playing a game with sexual content, but not something with a rape scene or even a mention of it, and it's not feasible to only play games made for children in order to avoid it.
If you could manage it, finding a way to either tie in trigger warnings to your age/maturity level ratings or come up with a tagging system for triggers would be great. A tagging system could work like the existing one, but without a limit of 10, and users could blacklist certain tags and either have games using that tag hidden completely, or they get a warning when attempting to view a page tagged with it, and a prompt asking if they want to continue, yes or no . The content/trigger warning tags could show up in the same place, under More Information.
Don't have much to say for the age/maturity rating system itself that hasn't already been said, but wanted to make sure I at least brought this up so that it could be considered.