Could it be possible, to use a mint offline? Like people who has ecash already from a mint could communicate with the mint, but noone would have internet?

If I understand correctly, when you use a mint, you anyway trust in it. If you do an offline transaction, you have to trust your peer too. If we can have offline mint usage, I still only have to trust the mint.

What do I miss? 

 You trust the mint but you do not need to trust other users. The receiver of a transaction needs to be online in order to refresh the token they received in order to prevent double spending. If there's no Internet, you need to trust the sender (or all previous senders) and the mint.