I know nothing about Greasemonkey, but based mainly on #3759 and #7064, I know the following should be possible:
One of the great drawbacks of MediaWiki is its complete lack of AJAXy features. To edit a page, you have to hunt down the edit link, edit the whole thing in a different tab, click preview, rinse, edit, repeat. I've actually stopped contributing because it is too cumbersome.
What I would like:
> A selection to edit is highlighted.
Everything about this besides the AJAXy no-refresh-blanks requirement seems reasonable to me, and I hope it is. It would also be a killer script, and I could tolerate editing again.
Requested script is so complex that (I believe) only one who really needs it will write it... Ask somewhere at wikipedia talk or at some wikipedians forum maybe.