that whole idea of "an open character development with progression" is a tad bit too undefined, for me.
leveling is a way of defining. i'm not saying, however, that advancement can't occur. but, i need quick, to the point, examples. no paragraphs.
Everquest Alternate Advancement system (AA)
Instead of "levels" you accrue AA points (akin to Perk points). The AA system is a hierarchy based progression system, meaning you have to invest X amount of points in Y tree, before you can move on to the next trees. The further you go, the more specialized the AA skills/abilities/modifiers are. For example, there would be a General Tree. In the case of Skyrim, all players would have to invest a certain amount of AA points in this Tree before they can move on to the next. The next tree might be Thief, Warrior, Mage with General AA abilities that help anyone playing any of these roles. Once a certain number of points are spent there, you might boil down into "Types" of Thieves for trees. Assassin, Ranger, Dirge, etc. You can take it even FURTHER and create another level, one that defines specific Skills or Abilities and modifies how these work.
The idea of the "Open System" is that there essentially is no cap to this. Characters seeking to make Hybrids would then invest in both and they might never reach the "final" AA tree but rather have points in 2 or even 3 of the second to last AA trees.
What this does is help give you specialization as you are really drilling down "what" and "who" your character is. Levels are not a factor in this equation and instead "attributes" are fixed. This gives way to Race possibly playing a larger role (as they could have varying attributes). I say possibly, because I am not entirely sold on this traditional RPG concept as it applies to TES.