Jack Staik wrote:

Example: Akane finds Ranma's Last Letter while he's out, where he
confesses his True Feelings (whatever they are). How does she take it?

Ranma wouldn't write such a letter.  No way nohow no sir.  He just isn't
built that way.

Example: Gourry Gabriev finds Lina's Letter, in which she writes of her
regret of never telling him how she felt. Does Gourry start acting
romantic? Does he get nervous? Does he foolishly say he read it, thereby
dying a quick death?

Lina might write one, but she wouldn't get all gushy over Gourry.  It
would likely contain burial instructions and a list of people who are to
be "gotten back" for her.
 

These are just two examples. There are examples, both canon and
non-canon, and they don't have to be romances. Is Asuka's Last Letter
addressed to her child, given up for adoption several years before? Does
Kasumi have a Last Letter, revealing her Dire Secret to her family?

They have to be people who would write such a letter, and they have to
have something interesting to reveal.  Sailor Moon might write one, for
example, but she would just go on and on about how much she loves her
Mamo-chan, and how much all her friends mean to her, and her little cat
too.  Then she would give away all her worldly possessions, then take
them back and give them away again because she forgot someone, and chide
Rei for not being nicer when she was still alive, and get all weepy and
teary and get the paper damp and mucousy, and then accidently flush the
whole thing down the toilet.  But she wouldn't say anything worthwhile.
 

Such tales can be comedic, dark, romantic, etc. They can be major
divergences, or have no effect at all.

Can anyone think of a 'Last Letter' tale?

Actually, I think I just wrote one ...

There are certainly possibilities there, but it's a fairly limited idea.

-- David