I don't see any reason for Ratchet/Nefarious to be as 'strong' as Ratchet is. Recent western characters all tanked, the rivalry isn't good or old or known...like people know Ratchet, but Nefarious? Not so much. It's probably going to be one of the bottom weakest rivalries in the bracket (although Sol/Ky would be worse).
Kerrigan/Raynor are combined stronger, older better rivalry, recency with Starcraft 2.
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Black Turtle did a pretty good job.