The idea behind trails is to skipp a bunch of levels so you don't have to do them everytime.
But they are a pain in the ass to do and most of us want to get rid of them as do I.
So here is my idea. make it so you can choose the Greater rift level you want to open based on the Highest clear rift for that character.
When you clear a normal rift you still get the trail keystone but when you open the portal as you to now you get to choose the level you want to keystone open.
The choosing of this could work the same way as when you choose the number of (for example) gems you want to make.
The picture http://imgur.com/lBqrGjH will explain how I envision it. (don't judge me on my pro paint skills)
Ofcourse if you have a level keystone it would stay as it is. This would only work for trailstones.
I think that the maximum rift you should be able to open should be roghly 10% lower then you currently highest cleard greater rift (much like trails work right now).
This way you always get to choose a level comfortable to you and when you change spec and or gear to test things out you instantly get to choose the level you want to try. You would only need to start at greater rift level 1, 1 time for each character.
I also posted this on the EU and US battle net forums