Ottumwa and Marshalltown would fit quite nicely in the Little Hawkeye Conference which is somewhat of a high-level 3A conference:
Pella
Norwalk
Dallas-Center Grimes
Pella Christian
Oskaloosa
Newton
Grinnell
Ottumwa?
Marashaltown?
Indianola?
I remember when that conference had Waukee in it (originally as one of the smallest members, but, yeah, haha, things change). I remember when I was in high school Boone was in it, too, but, yeah, there's one where the conference headed up and Boone headed down. I know Boone used to be 4A back at some point (like in the 1970s) and used to compete with the DSM schools in football at that classification, but that changed a long time ago.
I think you still want conferences for high schools simple on the grounds it makes scheduling a lot easier. Lot easier to make things work in a round-robin of pools of 8-12 than in some big grid of a couple hundred schools (or, at least, a couple dozen schools per classification).
I kind of think it's funny how the "super-conference" in Iowa is already falling apart because of competitive imbalances, awkward distances for high school students (which have to use buses) and families to travel, and just generally divergent goals and philosophies (lot easier for the DSM area schools to feel a connection to each other than, say, 50% of the DSM schools with 50% of the Avenue of the Saints schools). Might be a little test case for how some of the larger NCAA conference might end up? Granted, I think the SEC/B10 are probably stable enough at 14, but much more than that and I can see some N/S E/W or top/bottom divides starting to develop down the line for just these same competitive and geographical reasons.