I looked at a few pages that generate random stories an thought they were interesting, but was drawn more to the interactiveness of the letter generators. I also looked at a page that generates complaints about people, but it sounded like it just plugged in a name and threw out saved paragraphs so I didn't like it.
I ended up with two examples  that generate letters for people wanting to express a particularly strong sentiment.
The first is a letter of apology (http://net.indra.com/~karma/formletter.html)
The second is a letter to a stalker.(http://members.aol.com/deejpearls/stalker_letter.html)
Both of these sites do not require the "writer" to write anything of his/her own; instead, one picks phrases from a list.

They basically follow the following "letter for expression of strong sentiments, with a twist of ridicule" rules that I saw in them:

1- Greeting that sets the tone, adnmay or may not include the person's name.  Even if it does include the name, it still asks for a pick of an alias or title for the intended recipient.
2-An idea of the seriousness of the stiuation ( degree of remorse, degree of anger)
3-Reason the situation go to where it is( writer made a mistake, stalker is a jerk)
4-Reprisals expected
5-Wrap up/conlcusion/solution
6- Signature, no names.

All of the above give the choice of putting in phrases that express various degrees of strength of the feelings expressed, usually making it all absurdly funny(or attmepting to be).  Greetings go from sickeningly sweet to kinda insulting, situation/reason can be  serious or ridiculous, reprisals go from mild to violent, but most are just funny, wrap up gives a better view of whether the person means it or is just being ironinc(in the apology letter it could be either "kiss and make up" type, "lay blame on someone else" type, "I"m still a decent person" type)

The only space they leave for personal input is a rectangle where one can put one's own words to add to the above.
Examples
My assesment