Depending upon the campaign (especially common in the kind of 6+ year fantasy campaigns associated with D&D), Life Begins at Legendary
. (All of those are good, but the first is the relevant one; note the series was written long ago and the rules have had some significant relevant changes, so some of the examples and numbers will be different.)
With respect to game play and character power, Legendary groups are roughly on par with 10th to 12th level D&D characters. They're strong enough that early challenges are probably trivial for them, but weak enough that planar and multi-verse threats are "we need to complete a quest to get the tools to start this quest" territory. In-fiction the characters are A Big Deal: they run companies, hold titles, manage vast holdings, have widespread reputations based upon their deeds (real and exaggerated), are important people, and are known by important people
Starting Novice characters are roughly on par with 3.5 D&D level 3 or 4 characters - weak enough that a lucky critical from an orc warrior can put them down but strong enough to take on anything below a dragon. And like their D&D counterparts, they're impressive enough that they should be building a reputation.
Advancement doesn't change things, much. Each advance provides either an incremental improvement to existing abilities or provides entirely new options. Adding Sweep to a character, or New Power (quickness
) or Connections (royal guard), can have a dramatic impact on how that character interacts with the campaign and the character's effective power level. Increasing Fighting or Shooting or Persuasion by a die type (say, d8 to d10) isn't going to have much impact beyond a small increase in success and raise rates.
Generally, higher rank adventures will have more foes (still Extras), better equipment, and more difficult challenges (Social Conflict, Dramatic Task, Fear, etc.). The structure and hooks will be as epic or trivial as the campaign's tone dictates, but the big difference between a Novice raid on a Beholder lair and a Legendary raid on the same Beholder's lair is how many (and how powerful) minions and thralls the Beholder has to help him out.
As for adventure suggestions ... I don't have much. I ran a savage Red Hand of Doom
game about a year ago, and it was a blast. Lasted for about five months (originally for levels 5 to 11) and they were Legendary for the final fight, though I was giving them an Advance each session (with a couple of exceptions). Still, some great characters, lots of fun, and fairly easy to convert (though there were some tricky bits, like the climactic mass battle).
Good luck, and I hope that helped some.