Sorry for double-posting, but this is really a separate thought.
I don't want to exactly replicate English's case system, but I'd accept it with a little tweaking. Like maybe throw in a fourth case, say, instrumental, to unite constructions like "in Terris" tarnuunan, "by road", "with jam" sogooan, "smiled upon by the stars" hodjeron (note metaphor), etc.
This is actually a great idea.
So "-n" in the singular and "-(e)rn" or "-(e)ron" in the plural?