There are many excellent tunings for the 12 string guitar. You can do any alternate tuning that you’d use on a six-string guitar on a twelve string. Open G and D and DADGAD are especially nice as well as any that highlight fingerpicking patterns, such as CGDCGD.
You may also be interested in tunings where the octave strings of the twelve-string guitar are not tuned in octaves, but rather to something else. This is certainly something that can be done, but is extremely tricky to execute. Say that you tune your non-unison strings, the low E(sixth), A, D and G strings in fifths instead of octaves. So, high to low, you’re strings would be tuned like this:
1) E 2) E (same as string #1) 3) B 4) B (same as string #2) 5) G 6) D (one fifth higher than string #5 – same tone as 3rd fret, 2nd string) 7) D 8) A(one fifth higher than string #7 – same tone as 2nd fret, 5th string) 9) A 10) E (one fifth higher than string #9 – same tone as 2nd fret, 7th string) 11) E 12) B (one fifth higher than string #11 – same tone as 2nd fret,9th string)
Now unless you plan on only playing singe strings, you’ve got a lot to look out for. Playing an Em chord, for instance, would lead to the following Em9 voicing:
1) E 2) E 3) B 4) B 5) G 6) D 7) E 8) B 9) B 10) F# 11) E 12) B
While this will sound really nice, it may not be the sound that you’re looking for. Things get even more interesting as you go through the various chords you know. Take E major:
1) E 2) E 3) B 4) B 5) G# 6) D# 7) E 8) B 9) B 10) F# 11) E 12) B
Now unless you’re really adept at string muting, you’ve got an E maj9 on your hands here. Again, this may not be bad if that’s the sound you want, but it’s the sound you’re stuck with unless you’re very careful.
Now imagine if you were to tune to thirds. Your Em chord now has the following notes:
1) E 2) E 3) B 4) B 5) G 6) B 7) E 8) G# 9) B 10) D# 11) E 12) G#
You can see that this is going to be very dissonant. And this is a simple chord! Ultimately you’d have to make a different tuning for each set of strings and even that would become so complicated that you’d have to wonder if it were worth it.
A lot of the signature sound of the twelve-string comes from the paired strings being struck together. Tuning these strings as octaves makes this easy as pie. What will help you get a lot of interesting sounds is not necessarily learning new tunings as much as learning new and interesting ways to form the chords you know.
A few articles on Guitar Noise that might help further are Multiple Personality Disorder and Double Your Pleasure - A Guide to the Twelve-String Guitar.