Collaborative writing vs RPs

I guess such a topic existed before, but no matter.

Well, first of all general RP is collaborative writing already. Two people are creating a story together. But there are a lot of bounds and flaws like:
1) Be online and have free time simultaneously. If one has to attend other duties, the other has to wait.
2) Only adding new text, not editing old.
3) Plot has to be relatively easy, and conclusion (inflation/pop) must come quickly. Because night is short and people have to sleep too.
4) Decorations are often primitive, no beautiful descriptions, thinking on the right adjectives et.c. well everything is schematic except feeling of players. It may be fun, but then it makes RPs similar. No uniqueness.
5) NSFW. Chatting in the instant messengers attracts unwanted attention.
6) in 99% cases only two participants. Difficult to include NPC (non player characters) because it takes much time and thinking to write for two or more people.

So, how about some kind of collaboration story? It can be done through email.
Say: you receive mail, edit current story adding paragraph or two, doing fixes if you think thats needed, or even interspersing existing text with new sentences. Example: card game, say poker or black-jack losers inflate huh. One person gets losing card and describes his/her blow up till exploding.
You (other person) can add what did you see, said and thought right inside that text. Not after it. And this makes sense. If girl has already popped, depicting your thoughts on her plumping belly and bust is a bit too late. ^^
Fixes and comments can be italicized to distinguish them from rest of text.
Other possible ways: PMs on this site, but it is not possible to send them to multiple co-participants.
Online via shared text editors. My favorite is "Google Docs" fairly easy and powerful

Thoughts?