I was wondering if we were gonna include sub-classes into the game? I mean after lvl 10 there doesn't seem to be much advantage to keep lvling as a pure class, maybe include some prestige classes for each class, so those you wanna stay a "pure" class can keep lvling in a very similar class.
The thought had crossed my mind, although we've tossed a few different mechanics around. Keep in mind that, as your level goes up, you are able to use higher-rating actions with more frequency. For example, one-shot Slay abilities as they exist now are adventure abilities at 10th level and encounter abilities at 20th.
As you gain levels in a single class, you're also able to prepare more actions from that class. This has obvious advantages, since you're gaining a larger and larger repertoire of powerful actions.
Also, not to betray possible season three releases, but there's nothing to say the system can't scale beyond 10th level. Consider for a moment that Slay, Vanish, and Evade are not necessarily the most powerful effects that might exist, and then you might see a need to stick around in your class longer... ;)
Lastly, and most importantly, we're not fully decided as of right now, and we don't need to be. The way leveling will work, we won't be seeing characters of 10th level or higher until the end of season two, so we've still got a long time to figure out what will be the most fun for everyone.
(Of course, that's not to say we might not have something like prestige classes... )