I think Power vs. Score is 'broken' (in General)

It isn't making much sense to award Challenge bonus on power vs. score. It should be based on Power vs. Power. I don't think the player should be rewarded more because their opponents have a higher score, when in fact they might actually be weaker. This will also prevent the problem of scores being artificially inflated because characters unequipping and allowing weaker players in their own clan or whatever to farm score from them. Also, this makes Challenge bonus actually MEAN Challenge, because you are defeating characters that are more powerful than you and not ones that just happened to have been fighting someone else with a higher score.

Power vs. Power... At higher levels there is WAY too big of gaps..

Not possible, nor plausible

"because characters unequipping and allowing weaker players in their own clan or whatever to farm score from them"

this would be the dumbest idea ever... because if my score gets lower, I take more score than i normally would from those on my fightlist, eventually causing my challenge bonus to dip lower and lower as my opponents score does. And if my clan mate has a higher score, he become a better challenge bonus, and therefore a more tempting target by other clan people... End result, by unequiping and letting a clanmate attack me to inflate their score, my challenge bonus and clan bonus go down...

Power versus Score rewards Specialization though

Specialization = More Strats = More interesting