I have been playing slide guitar for about a third of a century. It's what I'm best known for DownUnder. I have played on many, many sessions over the years, album tracks, jingles and movie soundtracks. If you live down here in Australia, you hear me often on TV and radio commercials. 

Like most players, when I started learning slide, I tuned to an open chord, open G as I recall. I was soon sounding like all those other players I had listened to. Great!  But not great for long. It got boring to play and boring to listen to.  Plus, I had already started to decode the fretboard in normal tuning, and when I was in open G, all my landmarks were smeared out. I was playing, but I didn't know what I was playing. I didn't like that. Not only that, I found that the wealth of possibilities that normal tuning offers was gone. OK, I could get a big six string major chord, but so what?  I wanted to be able to state major, minor, diminished, sus4, augmented and whatever else might arise. Being permanently tuned to a major chord was a hindrance, not an advantage. 

I reverted to standard tuning. Now at least, everything was where it should be. Even though it was more difficult to mute out unwanted notes (more on that later), at least I was in familiar territory. As I mentioned before, I always like to know what it is I'm doing in the context of the piece of music. In other words, am I playing a 7? or a 3? or a flat 5? or whatever? 

I then tried a drop D tuning, but only tuning the 6th string down: D A D G B E. I settled on this tuning, and now I always play in this tuning, whether Iím playing slide or not, and no matter what key I'm in. This gives me the best of both worlds:

The bass strings ( D A D ) wind up being 2 tonics and a V. Neither minor nor major. I can grab those three strings with the slide and I have a nice low, thick open-tuned sound, which I can play against either minor or major chords. (diagram below) 

The gold bar shows where the slide should sit (directly above the fret), the R shows you where the root note is,  the 'X's, which strings you should mute.

Dom 7

I can't get a full three note chord for the Dom 7
but I can get the flavour with two, the 7 and the 5.
If you're good at muting, you can get the R note too.

Diminished

Again, I can only grab two notes, but I can slide
up or down 3 frets to reach the next inversion and get
that diminished sound.

Augmented

Same thing here. I can only grab two notes, but I can slide
up or down, this time 4 frets, to suggest the augmented sound.