#1
DGCGCD --- Does it already exist? ive never seen it anywhere

Its fourths for the first 3 strings then octaves. good for 5th chords on bass strings and the top 4 strings are a sus2 chord which changes only by changing the high D string.

Thoughts? Comments?
#3
I believe it's called Gsus4 tuning. I think i've seen it in a zeppelin song a long time back, but I'm not sure
#4
Wikipedia is your friend;

Apparently Jimi Page used it on the Rain Song.

As our Welsh friend mentioned it is the open Dsus4

If you listen to the song it actually works very well so give it some experimentation and let us know what you think
#5
Unless you tune to extend the range of the guitar, you can't really hear how a guitar is tuned, save for some slight timbre differences but 99% of listeners won't notice them. Just tune to what makes a song easy to play.