For those of you who are allergic to tomatoes, don't care for marinara, or are simply looking for an alternative to ubiquitous red sauce, here's an idea: braised onion pasta sauce.
This Food52 recipe, adapted from a another by James Beard, calls for just a handful of ingredients: butter, onions, sugar, salt, madeira, pasta and parmesan. Here's the basic gist: melt the butter, and add the onions; sauté them until translucent, then season to taste with salt and sugar. Cook onions further until they're jammy -- about an hour, then finish them with booze before adding cooked pasta. Shower with parmesan.
Based on some of the comments, you may need to turn up the heat to speed up the cooking process slightly, but if you like caramelised onions, this sauce sounds like a sure-fire winner. Once you're comfortable with the technique, tweak it to make it your own, swapping penne for the pappardelle, olive oil for butter, marsala or dry sherry instead of madeira, adding meat or mushrooms for heft -- you get the idea.
Braised Onion Sauce [Food52]