I am running IXCR this year and some of the Midwest sxs series races when able too. Midwest SXS is a new series that started last year. they have more of a Facebook presents. not sure about website yet but know one is in the works. https://www.facebook.com/midwestsxs/
GNCC would be the biggest out in the Midwest/ east cost
Heartland challenge is put on by the Iowa cross country series.