Okay, here are my thoughts.

> 1.By number of participant, sex and domination/submissiveness.

I've seen something like this elsewhere and they use:
capital M and F for adults,
lowercase m and f for teens,
b and g for preteen boys and girls.
Submissiveness is already shown by which goes before the slash and which after. So g/M would be a preteen girl dominating an adult man.
Consent would be shown by a separate tag anyway. M/F cons would be consensual with the male taking the initiative and F/M cons would be the same with the female taking the initiative (if you're not sure, just put M/F for the man doing the fucking).
But anyway, regardless of whether we add this age division or not, there might be all kind of combination, making such a tag useful for informing of what is going on in the story, but not so useful for searching by that tag.
This is why it might be wise to have a separate tag for sexual orientation, which should at least include options for straight, gay (male), lesbian and bisexual. But probably there should also be separation between bisexual males and females. Don't think one for straight males and females is needed, as the presence of straight sex in the story already means there are both a male and a female involved. You can use more then one tag in this category and they apply to the sexual acts in your story rather then the characters. Like, if you have a story about a bisexual man raping a lesbian girl, you should use the straight tag.
For the same reason there might be a separate category of tags for characters age, this might include: fetus, newborn, toddler, preteen, teen, adult and whatever comes after that I am not interested in.
The usage scenario will be that for example you want a story with straight sex, so you search the straight tag and then look through the M/fgg type of tags for additional details on each story.

Since I specialize on consensual and casual guro, I can't help but suggest a bit more detail to that.
Consent is a bit tricky since consent to sex might differ from consent to guro stuff. Ideally there should be a separate category of tags for each.
For sex there might be: con_sex, nc_sex, prostitution (it just occurred to me that sex for money kinda falls under level of consent), maybe something in-between.
For guro there might be:
willing - the victim is eager to do it, probably doing it on their own initiative,
consensual - the victim doesn't mind doing it, probably not on their own initiative,
reluctant - the victim does mind, but doesn't resist,
murder - speaks for itself,
execution - speaks for itself.

Then, it also occurred to me only when thinking of tags, casualness should be a separate category from consent. Casualness should describe the setting and consent - the characters' attitude. For example:
casual consensual - a girl being consensually snuffed in a setting where it is common to do so, like in some of my own stories, in Rosie's stories, who else? Right, Lord of Pancakes, we're still waiting for Tails.
casual non-consensual - a girl being roasted against her will in a setting where it is common to do so, like in a Dolcett world.
non-casual consensual - a girl from a normal world seeking to be killed, I remember there being a bunch of such stories on the old gurochan (back when it was gurochan.net), often the girl would organize things in her life beforehand, try not to leave traces, sometimes find a secret society of guro lovers who would help killing her in an entertaining way and help with organizing her disappearance too. The fact that what the character doing is not normal in the society is rather essential in such stories, giving her even more thrill and putting under risk of being discovered. That makes it fundamentally different from casual consensual.
non-casual non-consensual - any murder story.