So, me and a vocalist I'm working with are putting together an unplugged version of Knockin' on Heavens Door. Playing acoustic guitar without rhythm behind me is a bit new to me.

I'm looking for hints about playing cool rhythm guitar on an acoustic and maybe some kind of solo stuff. (I already do a few runs between the chords but maybe something longer would be nice)

Grateful for any advice =)

And still even though this is an acoustic thread: stay metal!
Well if you want an acoustic version of that song just check out some unplugged GnR, Eric Clapton's versions and Bob Dylans versions... its a heavily covered song so you should find ideas easily enough.

As for rhythm on acoustic guitars try and avoid solo lines if there are no other instruments as it will sound very empty....try and embilish the chords with incorporated licks whilst maintaining the pedal tones of the chord....if you know what I mean...

Just listen to some hendrix...or SRV unplugged...
This is a lie...no reason why...right from the outset
I play this (covering Dylan's version) and I play harmonica in the break between the first chorus and second verse and also at the end of the song. It adds a little depth and sounds good over the acoustic guitar.
This song is the "Chopsticks" of open mic nights, be careful. Can be dangerous territory.
god and woody guthrie
you'll find 'em both
in the grand canyon
at sundown