I'm making tamales for a party. I want to have a veggie option but want to do something more modern then the usual chiles+cheese.

(Not tamales, but still Mexican.)

My favorite is chiles en nogada. The version I do is roasted poblanos stuffed with marinated portabellos and cashew ricotta, breadcrumb-coated and baked. Then you smother them in a creamy walnut sauce.

We have a close friend from Guatemala, and for many years, every Christmas Eve she makes tamales for all her friends. Really a labor of love.

They're different than Mexican - wrapped in plantain leaves, not corn husks. One type she always makes are filled with chicken/peppers in an ancho-ish red sauce... but the other is filled with green olives, raisins, nuts, spices, etc. Very traditional from what I gather, so there must be recipes online. Not overly sweet or dessert-like. Seems like even if there were stock in the traditional sauce, you could adapt it to be vegetarian.

My all-time favorite tamale filling is braised bitter greens. I first ate that kind of tamale (actually, a tamalito) from a seller in the Xochimilco market. His were filled with quelites, greens that are indigenous to that area. If I were making tamales, I'd use a combination of Swiss Chard and broccoli rabe to approximate the flavor.

http://www.hotdamntamales.com/
Scroll down to read about their veggie options, great ideas for interesting tamales.

These are all making me hungry! I'll have to try a few of these fillings on Friday when we make the tamales.

I'm partial to jalapeno and cream cheese tamales

Mushroom and rice

Spinach ,cheese with tomate and cilantro. mmm.

I doubt it would be too dry, but depending on the them of the party etc, it might not be what the vegetarians at the party want to eat. If it's a mexican themed dinner I would try to keep the flavors at least somewhat within that palate. These Squash, black bean and goat cheese ones sound pretty good!
http://www.eatingwell.com/recipes/squash_black_bean_goat_cheese_tamales.html

or these bean-y ones from The Kitchn, which are actually vegan :
http://www.thekitchn.com/how-to-make-vegan-corn-tamales-85314