I've used both the "sus4" and "sus2" tuning for D and G:
DADGAD
DADEAD (EBEF#BE, which I got from Alex DeGrassi)
DGDGAD
DGDGCD
A couple others are Martin Carthy's "lower" version of DADGAD
EADEAE
and Orkney
CGDGCD
I don't use them for slide, but fingerpicking mostly.
Yeah, fingerpickers do a lot more with different alternate tunings than most sliders do.
"A cheerful heart is good medicine."
The last few recordings I did on lap steel, I wanted an "ambiguous" modality, neither major or minor. I usually use open E, but I lowered the third string (G#) down to E so I wouldn't have a third at all to deal with unless it was a deliberate melodic choice. I was mostly using the lap steel for a "choral" background, just 5ths.
EBEEBE
But that's about as far as I've gotten from open E or G for slide. I am interested in the C6 tuning(s), though. Just haven't had time.