I'm for class based progression trees.
Start as either Thief, spell caster, fighter,
end up as either: Specialized warlock (elementals, necromancy, etc)
or Ranger, Paladin, Barbarian Warlord
or Assassin, Thieves guild master, Politician
depending on the path you take
A forth low level progression chart would be cool too. Start off as an apprentice(anytype) and work up to the one of the three other trees as they advance. Would be good for the NPCs/ and henchmen etc.
It would not quite work this way. If I do it, each of the 8 Classes would get its own advance table. No tree-tables, really. If I did this it would allow me to tailor character advancement to each Class. I'd likely do a basic one for Retainers too.
This what I have right now:
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The Benefits of Experience
Every five experience points a character earns allows him or her to increase some capability relevant to his or her role on the warband. We call these character milestones Bumps. Every time the character reaches a Bump, his or her player should roll on one of the tables below and note the result.
Unless otherwise noted, a character may only roll a specific D6 result once, so if he rolls the same D6 number twice, he should roll again until he gets one he has not already rolled. Bumps that appear on a chart multiple times may be rolled that number of times. For example, if a chart lists +1 Vitality on a result of “1” or “2”, you could garner up to two additional Vitality from that chart.
Bumps that grant points toward purchasing new special traits or spells may be “banked” for future use. In this way a character can save earned points until he has enough to buy a new trait or spell of his choice.
Martial Bumps
These bumps work well for characters who earn their gold in the thick of combat.
Roll 1D6
1 +1D CCA
2 +1D RA
3 +1D CCD
4 +1D RD
5 +1D DR
6 +1D on KO Checks
Magical Bumps
These bumps are ideal for spell casters, and may only be rolled by characters with those Classes.
Roll 1D6
1 +1 Fate
2 +5 pts. Spell*
3 +5 pts. Spell*
4 +1D MA
5 +1D MD
6 +1D Arcane Post-Battle Checks
*This result may be rolled up to two times for a total of +10 pts.
General Bumps
Sometimes a character will want to develop along a track that is more general in nature, and these bumps fill that need.
Roll 1D6
1 +1 Vitality
2 +1” Move
3 +5 pts. Special Trait*
4 +5 pts. Special Trait*
5 +1 Fate
6 +1D Post-Battle Checks**
* This result may be rolled up to two times for a total of +10 pts.
** This result may be rolled up to two times but must be applied to different checks from the following list: Arcane, City, Mercenary, Resource, or Wilderness.
Non-Random Advancement
If players dislike the idea of random advancement, they could all agree to just select which bump they want from the tables.
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If I choose to expand it I would add tables specific to the 8 Classes and the Retainer:
Bard
Druid
Fighter
Monk
Priest
Ranger
Rogue
Wizard
Retainer
So 9 such tables. This would allow me to stick in the sort of advancements that give cool little abilities we've come to expect from certain Classes--the ranger's Favored Enemy, for example.
Some of this stuff you can already mimic on the front end with Special Traits, and the existing tables do allow for the slow acquisition of new Special traits. It all comes down to how much detail I think people want, and that I want as well.

We'll see. I want to run a few campaign games to see what this current level of advancement feels like.