Any suggestions for a seasonal dessert that will feed 40 guests?

You could make a gorgeous platter of seasonal fruit - chopped and maybe even put onto skewers - and make a BIG batch of homemade whipped cream. Melt down some good caramel and chocolate, and serve buffet-style. You get to have fun with the presentation, and everyone attending can determine their own dessert-guilt level!

How formal? Since you tagged fall I will suggest for that. Where I am in So Cal, it is 106F so we are not quite ready.

Pumpkin Cheesecake can be made day or two ahead and kept chilled.

Rustic Apple and Pear tart can be made earlier in the day but the component orange-almond cream and strusel can be made a day or two ahead (I don't trim into rounds, I make as a slab and cut into squares, serves at least 8 to 10 per slab. Serve with whipped cream or ice cream, outstanding recipe. Easy to make with purchased puff pastry.
http://nymag.com/listings/recipe/rustic-apple-and-pear-st/

Roasted apples with brown sugar, butter, cinnamon and a touch of cloves. The apples will be tender and the brown sugar, butter and juices from the apples form the most delicious caramel-like sauce. Serve in a bowl with a dollop of creme fraiche or spoon over vanilla ice cream.

p.s. And whipped cream for the fruit galettes

A bunch of easy fruit galettes: apple, fig, plum, pear.

How to Make Any Galette
https://food52.com/blog/6889-how-to-make-any-galette-or-crostata-in-7-steps

… and this might be fun to read if you decide to do galettes
5 Links to Read Before You Make a Free-Form Fruit Tart
https://food52.com/blog/7107-5-links-to-read-before-you-make-a-free-form-fruit-tart

My favourite dessert for larger crowds that is as quick and easy to make for six as it it for sixty is panna cotta. Cream, sugar, gelatine and a seasonal flavouring - done. For a smaller dinner party I serve it in small glasses or espresso cups, for a larger crowd I set it in take-away paper espresso cups. A big hit with male guest who often aren't into desserts is espresso flavoured panna cotta, served with a generous drizzle of Kaluha on top.

Bread Pudding with apples and raisins and a salted caramel or ButterScotch sauce is easy to do for a crowd and is always a big hit.

A spiced bread pudding with hard sauce or rice pudding or a couple of these delicious caramelized pear cakes: http://www.epicurious.com/recipes/food/views/Caramelized-Pear-Cake-103934 (I like to put toasted, crushed hazelnuts on top)

A crumble/ fruit cake

An autumn pavlova, with figs, apples and (frozen) blackberries. Honey. Cinnamon. Served in a big glass vase. Pretty stuff!

A croque em bouche.

http://www.foodnetwork.com/recipes/nigella-lawson/chocolate-cherry-trifle-recipe.html
I doubled this recipe and made it for a Christmas party. It was a beautiful presentation, rich, decadent and absolutely delicious. We made one adjustment (used very dense double chocolate muffins instead of chocolate pound cake), but I think otherwise stayed pretty true to the recipe. It was devoured.

If you live in CA or somewhere else that plums are still seasonal, the plum upside down cake from the Moosewood dessert cookbook is loved by all I've served it to, looks beautiful, and scales up really well. I've made it for about that many people using a few lasagna pans. If you don't have that book, here's a faithful transcription I found online:
http://eduardohueso.wordpress.com/anzac-cookies/plum-upside-down-cake/
I find the volume of butter/sugar mixture for the plum part to be overkill and usually cut it by half.

I think the slab pie idea is great. You could also make a few apple crisps, which are rather easy to put together quickly.

If it's a casual dinner, pumpkin cheesecake bars with gingersnap crust baked on half sheets are awesome.

Pumpkin pie made in 12" cast iron skillet will hold double recipe of any 9" pumpkin pie and serve about 20. You could repeat that, or do a doubled apple pie for second choice.

You could do a couple of large seasonal fruit crisps (they're easy to double or triple.) Or make a slab pie on a jelly roll pan, which serves many more than a normal pie. Martha Stewart's recipe with berries is on this site - but for a seasonal filling, you can do a single crust pumpkin, or apple with struesel topping, or pear/cranberry, etc: https://food52.com/blog/7125-martha-stewart-s-slab-pie Serve either of the above with vanilla ice cream.