At the moment if you’re at high rank but want to choose a class from earlier rank it would have a huge penalty compared to just choose a class in your current rank, this limit the customisations which is not nice =(
The problem come from the flat damage of the skills, like if you get a class at rank 7 and you think the skill would combo well with Pyro (rank 2) so you want to get Pyro at rank 8, but you can’t go back because the damage from Pyro is now too low to be usable.
I understand that it’s not good to just increase the scaling of all skills, because people may not even bother to use the higher rank skills anymore. But I want to at least when I choose any class at high rank, I should get the damage I deserve of that rank.
So I suggest a system that increase your skill damage based on “the rank that the classes become avaible” and “the rank that the player choose to get that class”.
Something like:
Skill damage = default damage + [(“Your current class rank” - “The rank class become available”)x10%]
Well the 10% maybe too much, so it need adjustment.
For example: if you choose Pyro (a rank 2 class) when you’re at rank 8 (your current rank) then the damage bonus would be [(8 - 2) x 10%] = 60%
So when you choose the Pyro at rank 8 all of your Pyro skills will have 60% more damage than when you choose it at rank 2 
Sorry for my bad english
Edit: fix some typos