One thing that's missing from the game at higher levels is true Solo quests. When you first get to the Harbor, there's a couple, and they're kinda cool. Then you have a few like the one from Lockania (not even a real quest) and then Giant's Vault on IQ island. But there's not really many others, and they're all the same quests for everyone.
How about some class-specific solo quests and a couple levels. Maybe with some themed mobs/encounters/problems in them and a nice end reward specifically for that class.
Some higher-level Examples (Remember, these are solo quests):
Barbarian - DPS challenge! You have to hack apart a series of constructs with massive HP before time runs out and they explode (doing massive damage in a massive AOE that *WILL* kill you and boot you from the quest). End Reward - Stacking Rage (5 min x1 charge), similar to Madstone Boots.
Rogue - Search, Disable and evade your way through a hallway that more closely resembles a living blender! With locked gates between sections that you have to pick (no keys drop, no mobs). Survive and find the treasure at the end! End Reward - Exceptional Search, Spot, Disable or Open Lock +5 item.
Cleric - A legendary warrior needs your help defending some innocent villagers from a rampaging horde of terrifying monsters! Keep him from dying through exhaustion, fatigue, level drains, massive DPS, poisons, diseases and curses! End Reward - Major/Superior Healing Lore Scepter of Superior Ardor IX (1 min. x3 charges).
Ranger - The air above the Marketplace Latrine is filled with giant, blood-sucking, fireball-shooting insects! No one can get close enough to swat them with their weapons, but your arrows can take them down in no time! Survive the encounter while making the Latrine safe again for your favorite vendors! End Reward - Stacking +50% ranged alacrity item (Maybe a quiver?)
Just some examples. Plenty more could be easily thought up for other classes, even make some specifically for Prestige Classes, maybe. Savants, Pale Masters and Arch Magi will all want different things, as will AA and Tempests.
I would add a little more flavor to the game, highlight some of the best qualities of each class/PrE. You could even make it a scaling adventure, like Crystal Cove, with the reward being based on the level chosen, so at each level as you progress/tr/change feats or AP, you can challenge yourself to see how much over your level you can handle and how well your build is doing.
Just something different and fun.