The two big categories in my mind are foreign campaign contributions (usually misdirected through others, so sometimes also fall out as fraud and money laundering charges) and failure to register as a foreign agent.

As a reminder, both of these categories have seen grand jury charges and subsequent guilty pleas by Trump associates. These have often paired nicely with 'lying to prosecutors' charges, which help clarify that they knew they were up to no good...

https://int.nyt.com/data/documenthelper/1886-indictment-giul...

https://en.wikipedia.org/wiki/Michael_Flynn#Investigations_a...

https://en.wikipedia.org/wiki/Paul_Manafort#Indictments_and_...