Inside lives a goblin that feeds on indecision.

Reminds me of the classic 1e adventures that had the players start as 0 levels and whatever they did in the adventure that would determind their class at the end. So if someone tried to cast a spell they may end up a wizard, or grab a weapon to fight, a fighter, or try and unlock a door, a thief.

I think a skill based system like in Runequest would be ideal for suchb a game. But you could do a 3e Modern with the understanding that classes would have to be introduces.

it is a lot like how you level up in the japanese final fantasy II

Pendragon characters start out fairly similar. You're a knight. You have a horse, a sword and a squire. It is much more the personality traits that distinguish beginning characters.

IIRC, when you attempt a skill, make a check beside it. Annually (Winter) roll for each skill that has a check mark, if you roll (d20) higher than the current skill level, advance by one.

Fudge-On-The-Fly starts your character sheet with only a name, sometimes general description, and a bunch of blanks to be filled in as the the game moves forward. Each player makes a choice for their character and fills in the blanks whenever the need arises. Once chosen the choice sticks. (Morgo summoned his spirit guide to point the way. His first of many shamanic abilities.)

Character creation is done as soon as you have a name (if you require one).
My group always found this style of play fun.

Reminds me of an old Graphic Novel that was about a bunch of genetically engineered soldiers. The main character's name was Friday (as in a Friday Job) he was more adaptable than the others and changed faster than all of his identical brothers. Still the other soldiers did learn from their own separate experiences but not to the same degree.

I think it would be easy to do. Only you have to convince the players to go for it. You just start out with one 1st level character, after the group goes up a level they (the PC's) can all choose to modify how they see fit. It might help to have a system that rewards actual skill use, but other than that, I don't think it would be to hard.

Just thought of something. You could use 3.5 D&D. Each character starts out as a factotum (from dungeonscape). Based off what they do, what they spend their "points" on, etc... dictates how their characters evolve. Sort of take away the multiclassing restrictions, and either you decide, the usage decides, or the player decides how the characters evolve. Evolving could be adding a level from another class (maybe a specific level not just a 1st level addition to best fit the evolution [i.e. the next level be what you get as a 5th level druid {wild shape, 3rd level spell, etc...}, even if you don't have any levels in druid]). Or add powers from monsters like aura of menace from hound archon.

That I would have fun playing. Mainly because I would want to know where my character goes next in terms of evolution. Ideas....

Another thing is sort of they find power crystals (or something) like in final fantasy V, that would give them access to new powers that don't have a good evolution (rage, powers from tome of battle, hexes from hexblade, etc...).

Also evolution could be dictated by not just actions done by the characters, but done "to" the characters. An over-abundance of being hit with fireballs, the player gets fire resistance. Constantly being charmed, the player can either resis charm or manipulate charms in some nifty way. I think there was a 3.5 source book that handled this, but I can't remember which. It was specifically with spells though, but could take ideas from it. It was acquired through feats.

Feats could be your way of "evolving" too. More I think about it, the more fun this sounds.