If I had to write tutorials.

All of them

Using 45* angles would be a really interesting idea for a tutorial that I haven't really seen yet
Creating the perfect layout is way too broad for a topic
Detailing your brushwork is very much different between styles, and it would mostly just be used to see how you would do it
Displacements are really interesting, and as much as i would love to see a tutorial for it, I think its been done before
Planning a map is also very different between people, but unlike details, this is an aspect where it would be really interesting to learn how it should be done properly
What is grayboxing?
Koth maps is all over the place. overplayed. simply just viaduct formula
PL maps has already been done (I think), but its not really that popular, and I personally have no idea how to make a good PL map...
AD has NEVER been done (to my knowledge) which sucks, because I had a really cool idea for an AD map, but I never really got it to a point that I liked it, and I feel that failure may have been what drew me away from the mapping community...

Preferably map planning, since it's rare that I see newer mappers do it effectively.

and also I can't do it either