The thing is, melee as it is now is encouraged to dip around and experiment with new class features.

That example with all the dipping?

Weaker than your first example, with just a few of the fun options that make playing mundane worthwhile.

By organic, I mean more of a "Fighters of Xth level deal d6 brutal strike damage; after Y levels that improves to d8s."

Then keep going up; there is no reason why they shouldn't be tossing around d12s at higher levels.

So, let them use d6s at 3rd level, say, then at 7th it improves to d8s, then at 11th they become d10s, and at 15th they become d12s. It's smooth, it encourages investing in Fighters (even if they only go 3 levels, there is a bonus feat and that magical Fighter level 4 at the next level...)