I've seen many threads in this place regarding

So I'd like to ask something different.

I want to write a quiet, acoustic love song. That's right, acoustic guitar only and a voice to go over.

I've listened to Cornell's unplugged sweden gig and Muse's Unintended, and I want to do stuff in that vein.

If anyone would advice me on scales, chords and other examples that should help me on this acoustic journey, don't fear to sound off and speak your mind.

Key of A with a meldioc structure is always soothing and I would suggest modes with a scale of your choice and Chord Progession wise just work out of the Key of A Circle Progession. Remember Its about the mood of the song so Don't add everything in at once!
I'm afraid this is the wrong forum for that question, and there's no forum that will tell you how to do what you're asking. Everyone has a different writing process but we all use the same tools, and therefor the majority of discussion is on the tools - not the goal or the process.

In other words, no-one can tell you how to write your song, and the scales, chords and other examples depend very much on what sound you want to create - and so your question basically boils down to -

How do I use scales and chords effectively?

Which is basically a lifetimes work and too broad a question.

What do you know already?
I've written seven 8 minute proggresive songs with acoustic sections. I'm working on 5 new ones. I know some music theory, but I go with what sounds well.
Also, I don't know much in the sense of i-ii-IV, I just play different chords and arrange.

I think I'd go with arpeggios and strums because it's acoustic guitar only, but can you give me any hints on what kind of chords to use? I just want an example to start with, the rest I can handle.

Think of the feeling you get when everyone at a party is tired, laying on couches. You'd want soft, warm notes to fill up the room right?