Wulf, are we talking 'redesign the game system' here, or 'change the way we play 3E?'
I don't have much for the latter. For the former, you mentioned the idea of stretching out the powers gained at 10th level so you get them at 20th instead, but still having the same number of nifty upgrades at each new level.
Consider the Book of Nine Swords that just came out. You get new tricks and special attacks at each level. You could add 'teamwork abilities,' which wouldn't be amazing on their own, but if you added them to each class, it could add a lot of depth to gameplay as you level up.
Imagine a hypothetical game that goes from 1st level to 5th.
1st - Young Kid.
You can only really succeed by teamwork, and only against modest threats (like an orc, or a tiny imp). Spellcasters only get mild utility spells, which only shine in niche situations (like levitate small objects, which is seldom better than just picking them up; or creating light, which seldom is better than just using a torch), and can only use them a few times before expending their power.
2nd - Everyday Person.
You're strong enough to handle a modest threat by yourself, or you can work together to handle tougher challenges (like an ogre, or zombies). Spellcasters can use utility spells several times without becoming too strained, and start to be able to actually do things people can't normally do (like levitate themselves, create illusions, or make people resistant to certain attacks), though not often.
3rd - Local Hero.
You're a hero in the eyes of normal people. You're strong enough to take on tough threats by yourself, and as a team you can handle mighty foes (like giants) or magical ones (like elementals). Spellcasters can alter reality (charming minds, shapeshifting, shooting lightning), though doing so strains them.
4th - Famous Bad-Ass.
You're able to kick the ass of creatures that could terrorize a town of normal people. As a team, you're capable of even slaying a dragon or fighting a demon. Spellcasters can alter reality with ease, and when necessary can summon magic with permanent effects (reshaping terrain, traveling in an instant what would normally take weeks of hiking, or seeing the future).
5th - Superhero.
You wrestle dragons bare-handed, casually discuss prophecies like they're everyday affairs, and don't use doors because you can teleport through walls. You still need teamwork to handle the occasional horde of demonic mind flayers, but the warriors can fell hundreds of warriors, and the mages can control time and space, and even bring back the dead.
I'm sure in each of those categories you could have several stages of power. Or even if you didn't, you could just have a wide variety of abilities to get at each level, X of which must be mastered before you can go to the next level. Like in Boy Scouts, when you had to have X merit badges before you could get Tenderfoot, 2nd Class, 1st Class, Star, Life, or Eagle?