Spending is obviously the big one. There will be bitter battles as the Dems try to leverage their control over spending to stop Trump’s agenda and enact their own. The GOP better get the wall locked in during the lame duck.
The other one to me will be Russia particularly, but investigations in general. The Dems will shut down the spygate investigations, try to leverage Mueller’s report into an impeachment change, and then subpoena the hell out of the Trump Administration on everything else.
I don’t think it will stop Trump, but it will make everything much more difficult.
There will be no wall..
Ryan will hang around to be sure of it.