A respec cost cap?
Yes! A lot of people are still working out the in's and out's of their skill trees and the prospect of an ever increasing cost is terrifying as R&D person within my guild.
A second role/multiple roles?
Yes. The system within Rift was excellent and especially useful for smaller guilds, where if the same person was a raid healer but dps in every other aspect of the game they could swap roles to accomodate. If groups were missing members everyone would shuffle their roles to get the best possible team. The utility of it is to obvious to not implement and I really doubt there is one negative to the system.
Switch advanced classes?
No. The AC's are seperate classes within an overarching class. I could foresee many issues with loot, skills and balance (Espcially when some classes would be able to fulfill every role as opposed to others, like JK/SW with 1 Tank tree and 4 DPS?) arising from people swapping ACs. It isn't the same as swapping roles either, which is simply moving points around within your skill tree as opposed to switching the entire AC where you would be changing your gear completely, the entire skill tree and set abilities.
BUT, and I emphasise the but, it should be made a lot more clear how important the decision of choosing your AC is. I don't think the game made it clear enough just how important and final the decision was.