I loved D20 Modern and own just about everything in the series. Really was a cool game.
As for this idea and the discussion of attribute classes vs standard classes (soldier, spy, diplomat, whatever, etc.).
Why not simply make the core classes attribute classes (Strong, Fast, Tough, etc.) and have archetypes cover things like soldier, spy, etc. I think that leaves room for lots of interesting combinations without forcing someone to be too specific by choosing a class that defines not only their stats and general direction but also a play style (eg: soldier).
Slightly different would be to make archetypes a shared list. Similar to occupations but much more influential over the development of a character.
I think this is a much stronger and still very D20 Modern way of handling things. Off the top of my head I can imagine what a Strong Soldier is like and what a Charismatic Soldier is like just as easily.