So I thought about a language with core and non core cases marked by prepositions
1, and there is an obligatory topic at the beginning of the sentence that will always get stripped of all adpositions and you can onyl see it's a topic, because it is unmarked. Let's look at some made-up examples.
(1)
Krikr nom iyunsi puk ak blab.
letter NOM 1PL receive ACC anwer
'We got an anwer to the letter.'
We have 'letter' as the topic, no preceding prepostion, followed by the subject '1PL' which is preceded by the nominative preposition. The verb 'receive' is then followed by an object letter, that is preceded by the accusative preposition. The topic is interpreted as the thing that was answered.
This sentence is equivalent in meaning (semantic, but not pragmatic) to another sentence
(2)
Iyunsi puk ak blab gen krikr.
1PL receive ACC anwer GEN letter
'We got an anwer to the letter.'
Now the 'letter', that was the topic in (1) modifies the 'answer', the 'letter' is integrated into the whole sentence and gets it's own preposition (yay!). Note that I used the genitive preposition, but it could have used any case really. Also note that the subject '1PL' has lost it's nominative preposition (Oh No!), because it became the topic. I really like the idea of the subject being the topic often, and then only getting a nominative if there is a preceding topic. I also like the idea that the preposition is somehow not able to survive if it would be initial in the sentence. The sentence boundary like to crash prepositions, hehe
What do I mean by adposition? It should be independent phonologically (be treated as a word by phonological rules and receive it's own stress), it should have at least one syllable. And it should come at the beginning of a noun phrase (see (3), not just before the noun (in the big house instead of the big in-house) . I also think it should be stranded when you form questions (e.g. what are you looking at) (see 4). Of course that means the language would have to have wh-movement. Not sure though if the topic or the wh-phrase would come first in a sentence.
(3)
Krikr nom iyunsi puk ak laa blab.
letter NOM 1PL receive ACC long anwer
'We got a long anwer to the letter.'
(4)
Krikr hee nom iyunsi puk ak?
letter what NOM 1PL receive ACC
'What did we get to the letter?'
The whole thing gives me a kind of Japanese+Oceanic syntactic vibe. I like that