Guts isn't going to do much to deter the Secondary. As I've said sine the beginning of the CAP project, there are a lot more secondary effects than Statuses! We learn nothing by giving this Pokemon Guts. The whole concept of this CAP is "Stop the Secondary" The purpose of CAP is to learn things about the metagame we don't already know. How does Guts do either of these? Guts will "stop" status effects, but does nothing for the huge part of the list that isn't status effects. It also teaches us nothing new.
Guts is just a horrible decision all around. The only people push for it is because they have forgotten completely about the concept of our CAP. We know how to stop status effects, but we don't know how to stop a large sum of other Secondary Effects and that's what we should focus on. Status effects are a problem that has existed since RBY. Why don't we focus on Entry Hazards, and Taunt, and Encore, and the plethora of Secondary Effects we should be addressing. Guts gives our CAP boosted attacks and thats it. We'll get a stronger pursuit, and that's it. Please remember the Concept and not just push for the ability you like best. Guts is cool, we get stronger, but it won't teach us anything, which is the point of CAP.
Alright, then:
Stoic: The user is not affected by Taunt, Encore, Sweet Scent, Spite, Pain Split, Curse (if used by a Ghost-type), flinching (note: this is debatable,) Torment, Charm, Growl, Tail Whip, Captivate, Attract, Tickle, Trick (also debatable,) Memento, Grudge, or Fake Tears. Does not affect Choice items. (May or may) Whether they affect Baton passing to/from CAP9 or not depends on what's easier for Doug to program.
Flavor, ingame description: Stoic: This Pokemon is unaffected by emotional or directly pain-based attacks.
Many of the effects listed above are for "flavor" aspects of the ability: the idea being "Which moves would be unlikely to affect Mr. Spock from Star Trek, or a highly genre-savvy villain?" On the other hand, not all concept arts look very stoic. Another thing: A big secondary effect is "stat drops" and I noticed a lot of them involve emotional references. Why not have a stoic Pokemon that resists them? (We can discuss whether to include flinching and Trick in that description later. I don't know if it should negate Intimidate or not, though. Switcheroo is exclusive to NU, and I highly doubt everyone will start running Iron Ball Switcheroo Lopunny in order to subvert this guy.) Obviously this ability is very powerful passively, negating many secondary effect moves at once (note: Sonic attacks are negated by Soundproof, hence Screech and Metal Sound are not included,) but it isn't going to cut it against Scizor, Gyrarados, and other Pokemon who don't use secondary moves. So in other words, this stops the secondary in ways other than status, as you suggested, Pink.
I say this because upon reading Pink's argument, I noted that it would be good to counter. We already have Machamp and T-Tar, so I think that Stoic would be a nice way of beating a lot of the secondary-using metagame without completely breaking this guy, as he's still vulnerable to Burn/Toxic/Sleep/Freeze (which we can stop and know how to stop,) and his ability is useless against most sweepers. (No OU Pokemon sweeper besides Jirachi and Togekiss suffer much depending on if Stoic negates flinch or not, and if we give it a high speed stat anyway we won't have to worry very much at all about flinches.)
-Torterra-Infernape-Feraligatr-