Dragon's Dogma and Kingdoms of Amalur both have very different classes that you can change to and from easily.
It's already a boon in Dragon's Dogma, but with Amalur it's more than welwome since the game is so fucking long it would be ages before you beat it and create a new character.
They are both excellent action-RPGs, in fact easily among the best on the market.
An edge case is The Lord of the Rings: War in the North. You can actually respec but each character has an ideal build so it's more useful as a fix than as an experimentation tool. It's more interesting to switch to another character and rebuild them from scratch. They have overlapping roles, but it's still interesting to try them out, and see unique dialogues and class-specific hidden areas.