Interesting ravioli fillings?

Fava bean and mascarpone. (Check out Keller's recipe, it is really nice, although I find it better without breadcrumbs http://www.epicurious.com/recipes/food/views/fava-bean-agnolotti-with-curry-emulsion-105751 ) Peas and mascarpone with a touch of lemon zest. I frequently sub ricotta with mascarpone, because I am not a huge fan of ricotta texture. Is seafood OK? Lobster ravioli with tomato cream sauce are lovely.

As long as you pair sauces with fillings I would just play around with sautéing or roasting a conglomeration of vegetables that are fresh and seasonal. I make purees all the time for savory danish at work and I often just take something like corn, eggplant, beans, root vegetables (parsnips, turnips, etc.), artichokes, mushrooms, peas, etc. Cook them with a bit of onion, garlic, olive oil, spices, then puree it coarsely or smoothly in a food processor adding in things like hard cheeses, nuts, herbs, sundried tomatoes, lemon zest, etc. Add warm liquid (stock, milk, water) to give yourself a proper texture. You can pair the filling with a fresh cheese like ricotta, but its not necessary.

Its a very loose definition of filling, but I don't think food needs a "theme". This gives you the freedom of playing around with flavors that you like and what's fresh at the time.

My local fresh pasta shop does a Gorgonzola and spinach ravioli that's very good. Also a potato leek.

About ricotta - it's a part of many fillings, even when not the dominant component. So assuming you mean no 'plain' ricotta filling, this Mario Batali spring pea/lemon ravioli might suit: https://www.eataly.com/us_en/magazine/eataly-recipes/spring-pea-ravioli/

I recommend trying cream cheese instead of ricotta if you're looking to switch things up. One of the best raviolis I ever ate was from the Vermont Pasta Company - they had a salmon and cream cheese filling that was ridiculous. I realize salmon busts the vegetarian thing, but maybe try it with some other filling? Mushrooms or spinach like Sam recommends would work well. Now I'm hungry and it's not even six in the morning...

Mix and match from other dishes you like, keeping in mind how you intend to serve the ravioli...in soup, sauteed or baked.
Examples:
Potato onion filling, saute the ravioli, serve with mustard & dill (as appetizer or side dish).
Beet filling with feta in a sauce, or beet & feta combined in the filling (baked or in soup).
Eggs (if lacto ovo ok) in center, quickly poach so egg is still runny, serve immediately in soup
Fruit-nut filling for use as dessert with greek yogurt or sour cream topping
Etc, enjoy

How "Vegetarian" No Ricotta? spinach and riccota are great.
Would a sweet potato be too close to pumpkin in flavor?
And then you can go to mushrooms, onions..Eggplant would be good but frankly just one egg plant very large.
Okay....a Japanese egg plant, with mushrooms..and use a Dashi/miso broth with seaweed garnish.