Is there an idiom for "I can't wait", or how do you say this? Is "sperare male possum" too much Portuguese, or does it make sense? Would there be a better (or classical) way to say?
Sperare male possum does not convey the meaning. If anything, it would mean something like "I can hardly hope", "there's no reason for me to hope".
I'm not aware of any equivalent Latin idiom to "I can't wait".
The verb gestio might possibly come close-ish at least in some contexts (that's typically used with an infinitive, so it would be for "I can't wait to...").