Off the top of my head, I can’t think of a word in English that ends with “is” while being singular, only plurals and uncountables come to mind, so I can’t really follow the examples of other words. What makes it even weirder, I’m not sure how to pronounce Illinoises… Would it be as written, or as if an Illinois was pronounced by someone who has never encountered it before? Illinoi are also meh, since now plural looks as a singular and the other way round.
I was thinking more like a split into North Illinois and South Illinois. I think we’d have to see South Illinois standing on its own before any mergers with Iowa or Indiana are on the table. Wisconsin or Michigan… I do not see that happening, no.