Little guy woke me up so here goes:
# 4 - Par 5 600 yards - Visually intimidating tee shot with a blind second shot to a drop down landing area. From memory, I think of the 4th green being tied into the creek as a kid. I believe there were many threads on those two bunkers in front which I believe were added by Fazio.
#9 - The damned up pond area of the creek has always been there, with a front pin it is in play. There actually used to be a pond at the bottom of the quarry on #17 which was never really in play. It was a mosquito heaven and they took it out in the 80's I believe. Today it is just tall wavy grass.
Trees at Merion have never really been in play. Fazio took out a bunch around 16 in the quarry on the right side, but it was more esthetic than playing. The only trees than come into play unless you hit a really bad shot are:
#2 - protect the cars from tee shots on the right side
#7 - protect the houses from tee shots
#12 - protect the dogleg on the right, they need to be there and always have
#1 - Now with modern play, I think it would be interesting to take out the trees on the right which visually block the green from the tee. Let's see Tiger and company go for that #1 green which is 330 yards or so via direct shot. Lots of OB right and deep sand to start a US Open.
Merion has never had a tree problem.