Im actually kind of hoping the appeals in the case (i.e. Don Mcgahn) eventually reach the supreme court and the high court supports the constitution in that defying a congressional subpeona citing executive privilege is in and of itself obstruction of congress. But the way those are jammed up in the courts, i don't think the high court will make a blanket ruling so it will have a trickle effect.

Considering guys like that have gotten removed from their roles in a lot of cases, congress won't have a case for impeachment against anyone, and things will seemingly work themselves out in courts sorta after the fact.

Best case scenario is I think for any FUTURE president not to be able to defy a congressional subpeona, as is built into our constitution, but that people make their decision on trump in November.