Guitars are a great object for tuning experiments. The Viola Caipira traditionally has a variety of different tunings. To that repertory I would add the one I learned in Portugal, used in the Viola Amarantina and Viola Braguesa. I was actually surprised that this tuning is not known in Brazil, or at least I have not found any mention of it anywhere. The original tuning goes(from lowest string upwards): D A B E A, or the same 1 tone below: C G A D G. Now the Viola Caipira is slightly larger, so given this change in string length, the tuning goes down a fourth: A E F# B E (or G D E A D). It works quite well because of the fifth in the bottom two strings. Chords of I,IV and V are very easily placed:
Here's a little action on the Viola Caipira:
Just now, looking at the common tunings of the viola in Brazil, I see that one arrangement is very close to the Portuguese tuning I use. That's the 'boiadeira' tuning, which goes G D F# A D, so it only really differs in that by displacing the second between the 3rd and the 4th into a major 3rd, we now have the chord on the dominant in the middle 3 strings. I suppose this makes the V - I very easy, while the IV degree is not so comfortable perhaps. I'll have to think about other chords, but on the whole, I prefer the one I use. The open strings make up a nicer 6th-9th chord, instead of the harsher major7th-9th in this 'boiadeira' tuning.
Another tuning I think might be quite good is the "rio-abaixo": G D G B D, which has a simpler open-string sonority, with the I triad in the top three strings. This looks better for modal stuff, as the V - I voicing is not that clear on the fingerboard. It should allow great play on G major modes, Mixolydian, Ionian, Lydian. The open B makes it a little awkward for minor modes, I suppose (well, I might get told otherwise, but that's how it looks.)
More later... this is an evolving story.