We have a mystery soup or vegetable for xmas. Any successful ideas?

Parsnip soup is a genius idea. Or try pureed parsnips, or celery root.

How about a roasted parsnip soup with cardamom

A creamy, buttery cauliflower puree looks like mashed or pureed potatoes: http://www.foodandwine.com/recipes/hot-buttered-cauliflower-puree

This soup is surprisingly good and simple to make - I would recommend starting with less mustard and brine then adding to taste, but otherwise following the recipe and allowing the toast to just start to burn: http://www.washingtonpost.com/pb/recipes/toast-soup/14260/

I'm totally in love with this soup: https://food52.com/recipes/8186-butternut-squash-and-cider-soup-1993 It could fit your definition of "mysterious" because you can't tell what it is by looking at it. Is it carrot? Sweet potato? It's also pretty light and sweet-tart. Even my husband likes it, and he's about the pickiest person I know.

kohlrabi served in a salad...batonnets or discs - unusual, but easy to accommodate to first time eaters - crisp, fresh, nice.

I only had Roasted Cream of Chestnut soup a few times but I think its holiday worthy.