Without directly increasing their damage, how would you buff a "niche" character?
As in someone with unique abilities or utility that is remarkably (if not only) useful in very rare circumstances IN COMPARISON to other champs. This means champs like Iron Fist, who is useful in most scenarios, have much better counterparts (Medusa for armor break, SS to nullify buffs, She Hulk who has identical base attack rating, etc).
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Personally, I'd go Crossbones. I would:
* Increase Overrun's duration by 1/4
* Decrease its cooldown by 1/4
* Make every 7 or 8 furies combine into 1 permanent passive fury (indirect damage boost)
* Increase odds of sig ability activating either during overrun or based on # of furies