Now, I've heard tell that there are already plans to change the crusaders to make them all useful, but I figured I'd suggest another way because why not. My suggestion is to make their last upgrade unique. What do I mean by this? Simple.
Not every crusader can/should be a DPS god, so some have to be supportive in one way or another. But as it currently stands, some are neither (Looking at you jason). So, why not give them an added bonus on their final upgrade, that also increases every 25 levels.

Bushwhacker - Every 25 levels it increases the effectiveness of Swordplay by 1.5%.
Jim - Every 25 levels it increases either Buddy System by 1.5% or Sharpen Party by 0.65%
Werewolf - Every 25 levels increases the Lone Wolf ability by 1.25%.
Sasha - Every 25 levels increases Bulwark by 0.75%
Hermit - Every 25 levels increase the effectiveness of Craziness by 1.15%
Kaine - Every 25 levels increases the chance to find a silver chest in a boss level already completed by 0.5% or increases the effectiveness of Magnify by 0.1%.
The Princess - Every 25 levels increases the effectiveness of Firestorm by 0.075% or the effectiveness of Ignite by 0.8%.
Jason - Every 25 levels increases your critical chance multiplier by 0.75.
Artaxes - Every 25 levels increases your critical damage multiplier by 0.25
Thalia - Every 25 levels increases his health by an extra 25.

and so on and so forth.
So yeah, that's basically it.