Thinking about Esperanto's case system, if I saw that in a natural language, I would think it was rather odd.
Esperanto only has two cases: accusative and non-accusative. The non-accusative, on is own, is a subject form, while the accusative is a direct object. With prepositions, either of these cases can be used. The non-accusative form is the default form used with preopositions, while the accusative form is used to indicate motion. For such prepositions, the non-accusative is used to indicate location.
Example: en domo: in a house en domon: into a house
This feature is of course not that unusual. German uses its accusative and dative cases the same way (though some prepositions always use one and never the other, you have to memorize which case each preposition calls for).
The odd thing is that the language only has 2 cases. I have a hard time imagining how such a system could develop.
Languages with such minimal case systems develop out of languages with more elaborate case systems. Over time, the various cases merge and you end up with a language that for some reason has a small number of surprisingly versatile cases. In the case of German, the reason they use the accusative for destination and dative for location is because these two cases merged with two other cases called lative and locative respectively.
But imagining something like this happening with Esperanto is difficult. Why would the dative fuse with the nominative? There are languages with two-case systems, but the only ones I'm familiar with have one case for subjects and direct objects, and a second that doubles as a dative and possessive case.
Is there really any natural language out there that uses a case system just like Espernato's?