I'm one of those strange souls who actually favours Proficiencies in ACKS, because I like skill systems in all my RPGs, regardless of genre. Given I like to use ACKS for historical games, skills are of even greater significance as a differentiating mechanism, when there is no magic, fantastical races and so on. I should also note that in my historical games, I cap level-based advancement at 5th level, anything more than that distorts the differences between experienced sorts and normal people too greatly and strains versimilitude.
What strikes me about the current system is that it doesn't give enough Proficiencies (unless your character has high INT), and the distinction between Normal Human and adventurers is somewhat arbitrary. What happens if a Normal Human gains a class level?
There seems to be a relatively simple fix as far as I can see: treat everyone as though they were a Normal Human first, and go from there. Thus a Normal Human (or 0th level character) gets four General Proficiencies (or two General plus one from a Class list) plus their INT bonus in additional General Proficiencies.
I have in mind the option of offering generational play for my games, thus you can move down the ages and use the descendants of the original characters, who might start out as youths. If we need an age breakdown, the first General Proficiency is gained aged 0-12, the next two (or a Class Proficiency) aged 13-17, the last General one at age 18. INT bonuses are spread out between Youth and Adult, depending on whether the bonus INT makes a difference to the total modifier.
Then at 1st level, player characters gain either two more General Proficiencies, or one from the Class list. This signifies the change brought about by gaining class levels.
Now the question remains as to what happens after that. Normal Humans gain additional General Proficiencies at 5, 15 and 35 years in their trade. I wonder about changing that to 5, 10, 15, 20, 30 and 40 (if you live that long!) years, with the option of gaining a Class Proficiency after two intervals.
Which leaves us with adventuring types. As before, I cap at 5th level, furthermore, I don't award XP, levelling happens at either appropriate-seeming junctures or the passage of enough time. I'm thinking Class Proficiencies (or two General) at 3rd and 5th level, and one General Proficiency at 2nd and 4th. They ignore the progression above as long as they are active.
Which would give an overall schema like this:
0th level: 4G (or 2G+1C) + INT bonus in additional G
1st level: 1C (or 2G)
2nd level: 1G
3rd level: 1C (or 2G)
4th level: 1G
5th level: 1C (or 2G)