Yeah, I presented this months and months ago and I still think it is a good idea.
So, I got this idea after being inspired by another thread. If we think of archetypes like Arcane Trickster as a pseudo-multiclass character, how about a simple multiclass-theme archetype to replace multiclassing (or offer another option)? My idea is this: you take the features for each class...
www.enworld.org
You are a cleric (wizard) with wizard as your subclass. You get level 1 wizard when you would choose your cleric subclass, level 2 & 3 wizard when you get your next subclass feature for cleric, and so on. By the time you get your last subclass feature, you are effectively level 7 in your second class. So, you would be a cleric 20 (wizard 7).
It fixes multiclassing while allowing you to still progress with your main class to level 20. In exchange for giving up subclass features, you are getting the first 7 levels of another classes core features.
The issue is some core classes are stronger than others, and not all the classes gain subclass features at the same levels (they are close though in most cases and could be adjusted easily IMO).