Sadly, I can't think of any truly open ending RPGs on the PS2. I can think of some that have variety and branching paths, but nothing truly non-linear.
However, if you're looking for action-adventure RPGs/Dungeon Crawler games like Champions of Norrath then there are several other titles in the genre you might like:
- Baldur's Gate: Dark Alliance
- Baldur's Gate: Dark Alliance II
- Breath of Fire V: Dragon Quarter
- The Bard's Tale
- Champions: Return to Arms
All but the Breathe of Fire game play similar to Champions, BoV is a more strategy based battle system.