Well, lying about it on security clearance forms afterwards, for one.

^^
I'm assuming Trump Jr filled out a SF-86. AFAIK lying or omitting information leads to revoking of security clearance and possible imprisonment.
Also, the Logan Act seems to easily cover Trump Jr's actions.

If Trump Jr can be shown to have offered any consideration in return for publication of the hacked emails that's criminal conspiracy.

The Foreign Election Campaign Act, and the Foreign Agents Registration Act might both cover such acts.

Should I keep going?