Just something that's been stuck in my head. 2e's multiclassing system got stuck in my head, first, and then the Experience as a River idea came up soon after. Then the two ideas got awkward in my brain and something strange came out that I kinda want to share, mostly because I'm interested in the potential consequences.
Let's make an assumption, first, that all classes can potentially result in T2 characters iff you have complete mastery of the system and leverage it to full effect, T3 for more typical optimization, T4 if you really don't optimize at all, and T5 if you are either really bad or actively trying to make your character terrible. T1 and T6 characters are generally not possible. For example, there aren't really Wizards in the game, but there are Beguilers, and Beguilers can potentially expand their versatility much easier than they can as written and might find a devastating spell combo that there is nearly impossible to defend against. It's not that the game doesn't reward system mastery, it's that there are no absolute trap options baked into the classes (much less are any of the classes themselves inherent trap options like, say, the CW Samurai).
Say we have character classes primarily work to provide a chassis similar to how they do in 3.5e (HD, class skills, etc), possibly with Talents like those in d20 Modern (selectable, feat-like bonuses that are exclusive to one particular class). However, experience rewards don't translate into levels, they translate into skill points, and the number of ranks in your highest-ranked skills determines your level. Specifically, if you make a fresh level 1 character, you can max out skills at 4 ranks each at start like you might expect, but you can increase the rank any skill to 5 after the game starts, 1 rank higher than the normal max. Once you have, say, 4 skills at rank 5, you go up to level 2 (increasing your HP, number of talents, number of feats, and other stuff). This would be standard across the board as opposed to one class leveling up after 3 skills and another leveling up after 5 or something like that.
You don't have to just power those 4 skills, though, you can broaden your range and pick up as many skills as you want. As a result, a party can be expected to be slightly uneven in terms of level, and experience distribution will be adjusted as a result: a higher-level character will receive less experience and lower-level ones will receive more. Eventually the idea is that the characters balance out in that you've got some that are very versatile and some that dominate in their specialized area.