I would have a play around in the D tuning, and also an open G.
Blind Willie Johnson's 'God Moves On the Water', in of course open D, like all of his.
I've been studying a few Robert JOhnson tabs such as 'Crossroad Blues' -, 'Come On In My Kitchen', and 'If I had Possession Over Judgement Day'. These are in Open A, which is just a Capo on the second fret of an open G, and was Johnson's most used tuning.
Technique tips- for a clean sound, dampen with your fingers behind the slide. Also slide down the neck as well as up, this can often be overlooked