epazote has a very distinctive taste but if you don't want to use it, I wouldn't subtitute it because as long as you have a nice pasilla flavored broth, you won't need it and other herbs won't necessarily make it good
From Wikipedia.... I would add a touch of one of these to taste to subsitute
Epazote is used as a leaf vegetable and herb for its pungent flavor. Raw, it has a resinous, medicinal pungency, similar to anise, fennel, or even tarragon, but stronger. Epazote's fragrance is strong but difficult to describe. It has been compared to citrus, petroleum, savory, mint and camphor.
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http://cals.arizona.edu/fps/sites/cals.arizona.edu.fps/files/cotw/Epazote.pdf
Mexican oregano is NOT epazote! Epazote is called Mexican tea.
Mexican oregano (Lippia graveolens)
Epazote (Chenopodium ambrosioides)
Epazote is used as a leaf vegetable and herb for its pungent flavor. Raw, it has a resinous, medicinal pungency, similar to anise, fennel, or even tarragon, but stronger. Epazote's fragrance is strong but difficult to describe. It has been compared to citrus, petroleum, savory, mint and camphor.