Since the slot seems to be collecting dust at the moment, why not a Dust Bunny? It would be DPS oriented with a skill or gimmick similar to Littlefoot's Growing Up ability where it scales with XP.

The longer it's sitting there, the stronger it gets!

If its appearance can change based on XP, then it could start out looking like a small harmless looking ball of fluff with bunny ears (smaller than Roborabbit) and get bigger as it gets more XP, eventually growing up to a giant muscular bunny that puts the Incredible Hulk to shame (with much the same silhouette). With a cap on how large it can be, of course. Maybe two times the size of regular crusaders? (His selection box would be the same size as other crusaders to prevent him from blocking other crusaders)

For balancing sake it wouldn't need to have an animal tag (since it's just dust), or supernatural, etc.

It could have an ability that worked similar to TurboTurkey's Short Circuit, where it buffs the DPS of crusaders in the row behind it (or around it) and gives itself a buff based on their DPS.

Or, if it isn't entirely DPS-centric, it could have something like a dust cloud passive that reduces the 'accuracy' of enemies (reducing incoming DPS), or works like a party-wide version of Littlefoot's Parry (temporarily stop incoming DPS in short intervals), or makes the enemies get lost and wander around a bit before reaching the party (maybe walk in circles or go backwards). Or if the dust is a choking hazard, then it could damage all enemies attacking the party (sort of like a more focused version of Merci's ability).