I'm going to use evazion's post as a template, incorporate subsequent tag suggestions, and add commentary:
bra_lift, shirt_lift, and bikini_lift generally expose the upper body, while dress_lift and skirt_lift usually expose the lower body.
Likewise, bra_pull, dress_pull, shirt_pull, and swimsuit_pull normally expose the upper body; buruma_pull, skirt_pull, pants_pull, panty_pull, and pantyhose_pull normally expose the lower body; and bikini_pull might expose either or both, depending on whether you're referring to the top or the bottom.
Similar to, but not the same as, the *_aside tags. Should probably be nearer to them (or vice versa).
This one differs from most of the others, since it's more about concealment than exposure. In particular, it's about using the shirt to cover for the failing of the bottom wear.
Tag Group:Body Parts already covers the following: armpits, breasts, navel, and ass. OTOH, some of these don't describe a body part so much as they describe the partial exposure of a body part: sideboob and underboob are probably fine, as is buttcrack (I think), and cleavage.
Likewise, I'm not sure if midriff refers to a state of dress or a body part.
I'm withdrawing my suggestions of down_blouse, downpants, and upskirt. Those aren't about states of dress; they're about point of view. I'd suggest a "points of view" tag group as well, but I doubt that there are enough tags to warrant it.
OTOH, these definitely belong here: they focus on the absence of clothing that you'd normally expect to find, rather than on what the clothing was supposed to be covering.
(Edited in response to NWF_Renim.)
The trick to this subset of tags is to avoid getting too specific: there's no difference between "no pants" and "no skirt"; but you might be able to get away with "no bottom wear" (distinct from "bottomless" in that one would indicate the total exposure of the lower body, while the other would merely indicate the absence of lower outerwear such as pants or a skirt).
strap_slip and unzipped are similar in that they're not about what's being done to particular articles of clothing so much as what's being done to the parts of the clothing that are supposed to hold them in place: straps hold the top up, while zippers keep closed the pants, skirts, dresses, and so on.
As mentioned above, these should probably be clustered with the *_around_the_leg tags.
These imply that something's wrong with the clothing. I'm thinking that they should be next to the open_* tags.
Nothing has been moved out of place or damaged in any way; but the clothing fails to conceal what it covers.
Not so much a state of dress as an activity that changes one's state of dress. Should we keep these?