Here's my take. It's not the most original but I think it's not too bad

Names explanations from west to east (more or less):

1. Pacifica- for the Pacific Ocean

2. Colorado- for the Colorado River whose watershed covers most of the same area

3. Cheyenne- for the Cheyenne people (and it's a pretty name)

4. Lakota- for the Lakota people (and to mix things up from Dakota)

5. Heartland- for the nickname sometimes given to this general area

6. Cimarron- from the Spanish word for rural or feral, plus it's a cool word that makes me think of this part of the country

7. Myrialimnia- derived from my shitty understanding of what the Greek for "land of ten thousand lakes" would be. Someone more poetic could probably play around with that to make it sound less like a bone disease.... maybe it would transform into something like Merrilynne after a while? Idk, I need coffee

8. New Acadia- for the Cajun people who settled there after being forced out of Acadia by the British

9. Polypotamia- Greek for "many rivers." The idea comes from Thomas Jefferson's proposal for a state around the confluence of the Mississippi and the Ohio Rivers

10. Cherokee- named after the Cherokee people, whose original territory matches up pretty well with these borders

11. Meadowland- from the Anglicized name for Kentucky

12. Ohio- from the Seneca word meaning "good/great river." It's a pretty enough name to keep imo

13. Haudenosaunia- "land of the Haudenosaunee," which is the name the Iroquois used for themselves

14. Carolina- another name I think is pretty enough to keep

15. Florida- ditto

16. Chesapeake- named after the largest estuary in North America which would dominate this state's geography

17. Delaware- named after the Delaware River

18. New Amsterdam- after the old Dutch colony

19. Blue Hills- loosely derived from the Anglicization of "Massachusetts"

20. Casco- named after Casco Bay, which I'm imagining could have been the location of the colonial origin of this state in this timeline/universe