The Haute Route is a classic. Rumor has it though that it is not exactly an ideal snowboard/splitboard tour, and actually that goes the same for skiers wanting to get a lot of downhill.
This was originally a "skinny skiers" route. I do believe over the years people have worked out ways to do this that make more of a downhillers route, but there is only so much you can do. Again, a classic, just have to make sure you have the mind set.
There are some newer multi day trails that are supposed to be of better quality as far as the downhill riding is concerned.