That's a bit of an odd question. Can you clarify what you mean by "...gets stronger ... without spending traits..."
A Level 25 cleric should be able to walk over almost any enemy in the game and easily complete both the Arc story line and the Sewer of Horrors. Unless there's something crippling your build? So what more do you want your character to do?
There are two benefits of gaining a level:
1) Per Level Benefits such as HP, MP (which are often tide to traits and skills).
2) Getting trait and skill points to use.
So if you eliminate trait points, you're down to skill points, which can be spend in countless ways depending on your class, build, guild, and playing style. So we're back to asking clarification.
If you're looking for a class to try as a second playthrough, warrior is an obvious choice to follow-up most cleric builds. They do more damage, but won't have the healing ability of a cleric.