@Antonis
I have a very quick suggestion. Not sure how easy...
Imagine you are writing HTML. You want to write most lines to a standard length, say around 120 characters, so the HTML produced is readable. If there was a vertical guideline in the window at 120 characters (that you could toggle on/off and set placing as an option) that would be a great help. Obviously this should only be used with monospaced fonts.
Note we aren't talking about chopping the line, just having a visual guide so that the line-ends can be made fairly consistent. Although an auto-return (on space character) when the guide is passed might be an interesting possibility too...

Generally I start with a good linelength, but by the time three or four pages have scrolled up, it's drifted longer and longer and I have to re-edit, which is tedious.