This is a good goal, and I absolutely support any author who voluntarily provides trigger warnings.
The problem I have is I don't know what's going to traumatize someone. There are things that may seem obvious to you and me, but a) there are people who aren't bothered by the obvious, and b) I will miss things. (I have missed things.) A useful, generalizable system is, it seems to me, both impossible and prohibitively problematic.
This. Some people may be triggered by the presence of dogs in a story (say someone was mauled by one as a child), or by a portrayal of living through a natural disaster. This isn't to trivialize the experiences of people who are triggered by the kinds of things more commonly associated with trigger warnings, like rape, torture, but it's also true that I can't say or dictate what people will be, or should be, traumatized by. It's a rare book indeed (at least in the genres in which I read and attempt to write) that contain nothing potentially traumatizing for some people.
The only option I can see is a very general warning, not unlike the ones used on television, "Reader discretion advised," or akin to the very general and broad movie rating system we use in the US. Such a system is pretty vague and not useful to someone who is, specifically, triggered by dangerous wild animals attacking humans but not by rape, or vice versa. Sometimes the blurb on the back can give the reader an idea of what kind of stuff is in a story, but what if the triggering event is in a scene that wasn't central enough to be alluded to in the blurb?
And that doesn't even get at the issue of where the line is between content that is merely upsetting or repellent to some sensibilities without actually triggering more severe reactions in those readers (say someone dislikes sex scenes or swearing for religious or other personal reasons, but not because they find them traumatizing). People often argue against trigger warnings with the assumption that some readers (or students or whatever) are simply being special snowflakes who don't want to be challenged or exposed to ideas they find repellent. But there's a difference between making someone relive a personal trauma versus simply making them angry.
Heat ratings don't negate the need for notes about triggering content. Romance is where I see the most content warnings. Those don't appear to cause any major problems for Romance readers or authors.
As for Horror, sub-genres act as a type of intensity rating, don't they? I expect different content in in Dark Fantasy than someone would in Splatterpunk (which I know to avoid ).
YA had databases. The ones I knew of were on Tumblr. The "Oh, no, think of the children!" scheme by Verizon appears to be shutting the databases and lists down in the place teens felt comfortable checking.
I think that with romance it probably works better. For instance, we all know that romances sometimes have rape scenes or scenes of rough or violent sex (that may be consensual, but still triggery for some readers). That kind of thing is pretty easy to mention in a general content warning. But other genres may have a much wider range of potentially triggery things.
For SF and F, I'd be fine with letting readers know if rape, torture, or graphic violence are included in the story, but again, it would be impossible to mention every specific subtype of violence that could be a problem for every reader. You're right that the subgenre will give hints too--grimdark versus high fantasy, for instance. Still, there is a lot of crossover, and libraries and bookstores don't tend to shelve by subgenre.