If I've got the Bitcoin client installed with some bitcoin in my wallet.dat, and I send some bitcoin to someone else, and there's no proof of any exchange so it's effectively a gift, is it possible for there to be a law that declares that as unlawful?
Dude, seriously, this is like asking "can i be taken to court".
Yes, there can be a law. Whether this is lawfull / constitutional is another thing, but never underestimate the power and stupidity of the political establishment.
For example germany
The current government has more laws revoked by constitutional court... than all governments since the end of the hitler era TOGETHER
You think that would be impossible - doing your homework etc. Obviously not. They are jsut sloppy and incompetent enough that enough slips through the craps. Make a popular law, then have it revoked as not constitutional.
So, yes, anything can be declared illegal.
Plus, child molesters and terrorists can justify anything.
Get the idea