How quickly could things unravel? - Shifting leveling up method

I've been thinking about working on a homebrew world inspired by where players make mechs or suits of armour that they pilot.

The level up change would be players follow a class progression but be able to mix and match subclass features to their mech build.

I want the feel to be heroic and like they are an awe-inspiring bastion at the end of the world so balance would be secondary. However, is this short sighted and going to break 2014 5e?

Thoughts and feedback appreciated.