Since the traditional mashed potatoes and sweet potatoes are already mushy, how about skipping the cream sauce on the green beans altogether? Instead, blanch them, then toss them into the serving dish with an interesting topping? If the beans are al dente, they can sit around for awhile and not lose texture - just get them into a warm serving dish when it's time to eat.
* melted butter with lemon zest
* a "gremolata" of finely diced almonds (toasted in a dry frypan earlier as an option), garlic and either lemon zest or parsley
* slivered onions and celery that have been softened in butter
Sprinkle on some sea salt at the table, and voila gourmet but easy update to Thanksgiving favourite!