One of the threads somewhere hinted that LL was thinking about adding a new "layer" for tattoos/skins. This would mean we would have the following layers:
Head:
- Base Skin
- Tattoo
Upper Body:
- Base Skin
- Tattoo
- Undershirt
- Gloves
- Shirt
- Jacket
Lower Body:
- Base Skin
- Tattoo
- Underpants
- Socks
- Pants
(and the meshed skirt, and - upcoming, according to this forum! - capes)
Now, the problem with this model is that the decision of what layer the clothing goes into is done by the creator - and not by the buyer. While base skin and tattoo have a different interface than the rest of the clothes, the problem is when you want to mix outfits from different clothes designers. For some reason, almost all creators of lingerie think it's a wonderful idea to have bras labeled as "jackets" and underpants as "pants" or so. Meaning that it's pretty hard to mix outfits so that they make sense. However, I can also understand that some people actually like to wear their underpants on top of their pants, so giving clothes designers the choice where the item should go is too restrictive.
In an ideal world, clothes designers would save all clothes they sell into different layers - so I could have a version of a top that is "wearable" as an undershirt, shirt, or jacket, depending on the rest of the clothes I'm wearing. However, since it's impossible to get the clothes designers to do that (they would be selling "several versions" of the same item for the same price...), I suggest a different idea:
Let "wearing clothes" work just like attachments. People would just have basically three types of clothes, corresponding to the three different clothes templates (head, upper body, lower body). When wearing them from inventory, you'd be able to select the layer you want it to go - precisely like the attachments! The system would "remember" the last layer it went to, so, the next time you double-click on it (or drag to your AV), it would be worn on the "correct" layer. You could even keep the current tiny icons to reveal which layer that item is currently "on" (ie. if I last weared my bra as a jacket, it would show in inventory with a red jacket icon).
This way, the designers could "suggest" an appropriate layer when creating the clothes, but the choice of which layer you are going to use is done by the buyer, not the seller. On copyable items you could even use the same piece of clothing on different outfits, on different layers.
If this means too much inventory redesigning, perhaps a slightly different approach would facilitate the actual coding. Clothes are just textures with special attributes. Instead of redesigning inventory, you would be able to open Appearances, drag, say, a jacket to your avatar, "change layers" so that it's displayed on the Shirt layer instead, and save it. If the item is copyable, you'd get a new copy. If it's non-mod, the original will of course disappear (no jacket any more) and you couldn't change the settings on Appearances, but you could at least save it as a different clothes item. I think this would be more than enough for the purpose, and limit the amount of necessary programming (imagine just adding an extra button called "Change Layer" or so). Inventory would work exactly like it works today.
Please note that I'm not asking for more layers
(I know, this would stress the "textxure baking" code) I'm only asking for a way for the users to have some extra flexibility. 5 or 6 layers of skins/tattoos/clothes are more than enough for all sorts of "special effects", I think. We just need a way to be able to decide which layers to use, since we can't rely upon the clothes designers for making sensible choices 
naturally all this would be for mod items only.
lol