I make a sauerkraut salad with chopped onions, a bit of garlic, caraway and pumpkin seed oil. If I am cooking sauerkraut, sometimes I add a bit of water, onions, garlic and chopped pineapple. I put it in a large dish to bake slowly in the oven til the kraut is tender.

When making a salad with sauerkraut you can add whatever spices you like, but to ferment authentic sauerkraut from scratch, good cooks in Eastern Europe add to the cabbage only shredded carrots, a few fresh cranberries, black peppercorns, a couple bay leaves and coarse salt.

Based on a great suggestion by BeijingRose in a comment to one of her recipes, I've been lacto-fermenting the hard bits of cabbage used in other dishes with scallion tops, garlic and then topped off with just a touch of rice wine vinegar, to produce what's sold in jars as "Chinese preserved vegetable," or "Chinese pickled vegetables." I add the stems to mustard greens when I have them. They're really strong (as in forcibly sinus-clearing, but in a good way), once pickled. In fact, I'm now lacto-fermenting all the hard stems and end pieces from Asian greens lately. They're delicious! And they only take a week or so to ferment nicely. ;o)

Since it hasn't been mentioned yet, I'm going to suggest ginger.

I love mixing things up a bit with my sauerkraut. Red chili flakes, coriander, cumin, caraway, juniper berries, and even whole cloves are great in sauerkraut (as are fresh chiles, grated apple, carrot, beets, celery root, etc.). I agree with William--don't mix all those together, but do keep in mind that sauerkraut can take a lot of seasoning, so whatever you choose to use, don't be shy!

Yes, definitely caraway seeds would be my suggestion!

Most common is Caraway seeds, which add an old-world flavor and certainly are delicious.
Also: Juniper berries, bay leaves, hot chilies, coriander are among the most common.
I wouldn't necessarily mix all those together. You might want to see how they influence the kraut before you add a lot.