There are two suggestions in the "Bugs and ideas" section of the Eshell 
manual:

   Allow "$_[-1]", which would indicate the last element of the array

   Make "$x[*]" equal to listing out the full contents of "x"

I think these would be pretty useful, especially for the "$_" variable, 
which gets the last argument of the last command, but if you give it an 
index like "$_[N]", gets the Nth argument of the last command. However, 
it's not as easy to get the second-to-last argument of the last command, 
or to get *all* arguments of the last command. So the above two 
suggestions would be pretty helpful.

Attached is a patch to do this. For the second suggestion, I took some 
liberties and added range syntax, so that "$x[2..5]" returns elements 2, 
3, and 4 (zero-indexed) of x.

I have just one question though: this implementation treats ranges as 
half-open, i.e. "M..N" is [M, N). I think that's the best way of doing 
things (and it matches how 'seq-subseq' works). However, "M..N" is the 
Bash syntax, which uses a closed range, [M, N]. Maybe this would be too 
confusing for users? I'm open to using other tokens aside from ".." if 
that would help. Maybe "M:N" would work? That's the Python syntax, which 
behaves the same way as this patch. Any thoughts?