I think the only real gamechanger here is the Elder Scrolls franchise. Skyrim is probably the most successful rpg of all time; if it's successor is exclusive to Xbox, then that's quite the loss for Sony. It's a much bigger deal than FFXIII or any other franchise associated with Playstation going multiplatform. While the other Zenimax IPs (Fallout, Doom, Wolfenstein, Dishonored, The Evil Within etc.) are nothing to sneeze at, I don't think they would truly move the needle in any serious way.
As for as a possible 'response', well, the only other publishers with an equally impressive catalogue of IPs whose worth should be in the same ballpark as Zenimax are Capcom and Square Enix. There's Konami too, but they don't do much AAA development these days, so Sony might as well just buy their IPs. But without additional development studios, who is going to make those games? I assume Sony's own studios are all busy with their own projects already, which is why such an acquisition would only make sense if it included new studios. In this regard Capcom and SE would have much more to offer. Between those two, Capcom would probably be the less expensive one, while SE's catalogue of IPs is stronger imo (and Dragon Quest would be a huge boon for the Japanese market).
Sony could definitely do it; hell, they could buy both of them, if they really wanted to. The question is if they think it would be worthwhile to do so. Personally, I'm not sure if it would even be necessary. It depends if those rumors about Sony moneyhatting major 3rd party games are true.