Is it possible to suggest existing projects (in which I do not participate) ?

I have few in mind like Unicorn engine (http://www.unicorn-engine.org/).

You mean post them as Apply HNs? No, but you could convince them to apply for themselves.

I mean a "Suggest HN" people can vote for with no obligation to Apply (nor for YC to do something, like a Show HN).

Not to get in the weeds too far, but I obsessively do this too. Have you tried CSS to make the input box look more like the final comment? I'd always assumed the current style was intentional and it's hard to say if it'd be a good idea, but just throwing it out there to see if it sticks. I know HN values its scrappy design.

I would be supremely interested (as a victim of this really aggravating compulsion myself) in an A/B on your end, to see whether people do this less if the box is visually very close to the final product. My gut says yes, and it'd be useful data for lots of designers. (Anybody reading that's tried it?)

Something unconscious about the flow of paragraphs, color, length of my lines, something drives me back to editing religiously until the software no longer lets me, on any site. It actually bothers me and I've already done it twice on this comment. (Make that five times.)

Haven't tried it. My assumption is more that the mind switches into a different mode once the comment is 'real' (because now it's public) and immediately sees different things. You're right that we could test that, but there are so many other things to test.