In my opinion, it's going to be Stephanie.
Although I'd prefer to see a feud between Orton and Stephanie, mostly because of his (Orton's) seemingly blunt, merciless, and heartless remarks/demeanor, her (Stephanie) joining Legacy would make more sense to me.
For me, the WWE did two things in the final segment of Raw with Jericho and Orton.
1. They contrasted the characters of Jericho and Orton as heels in the company. Orton being the one who, so far, supposedly sees no authority. Jericho will get on his knees and apologize when his job is in jeopardy, whereas Randy smacks, kicks and punts his boss in the face under the same circumstance (albeit his reaction at the end).
2. With Jericho getting on his knees and apologizing to the fans (disregarding the facial gestures), they provided a stepping stone for a possible face turn. Basically cleaning the slate of his antics over the past year or so.
With that said, Jericho, throughout his career (from what I can remember) just despises Stephanie McMahon. My only memories of these two together involve Jericho mocking and humiliating her with jokes about ****ing and breast implants.
Then there's the fact that when Jericho first returned, he said that he was going to save us and take the championship from the Orton the first chance that he got. Not that the whole point of his coming back was to save us from Orton, I think at that point he was talking moreso about the overall weak product, but his saving us from Orton was a part of the message that we got from his introduction nonetheless.
There's another thread on these boards discussing "Who will be Orton's Austin". Like a few of the people who have commented in there, I think that title is backward. Seeing the role that Orton is playing, I've thought and expressed that I felt if there were any comparison, Orton is more Austin-esque than he is similar to the Rock. Which asks a different question. "Who will be Orton's Rock". As far as charisma is concerned, the only person that I think is capable of the comparison at this stage (on Raw) is Jericho. He actually has verbally sparred with the Rock before, and he was able to hold his own in the battle of wits. Austin, to me was just more raw, which Jericho can be also, but I'd prefer it if they allowed Orton to continue to use that and run with it. Basically, if you were to take John Cena and Jericho and put them together into one person, it'd be like a "mini-me" version of what the Rock was in my eyes. If this storyline builds Orton up into what I think it could, I wouldn't be surprised if those two took turns going at him actually.
The prospect of Orton being fired by Stephanie actually came out of left field to me. With Jericho taunting her about her father's return and how her words meant nothing in comparison, I wouldn't be surprised if she panicked and looked to Orton to do the dirty work, in return for a favor. She did get a little weird when Jericho brought Vince up in the first place.
Both Shane and Jericho made reference to Stephanie's tendency to align herself with WWE talent, sometimes romantically, to cement her role at the top also. She's married with child now, so I'm not sure if they'd actually take that route or how HHH would feel about it, but she is a McMahon afterall, and when it comes to the WWE they'll do anything. Randy, on the other hand, I'm not so sure either. Although I guess an Orton/Stephanie relationship could also tie in to the Kelly Kelly/Kane storyline and make a little more sense out of that. But is Kelly Kelly deserving of the storyline push? I haven't watched enough of her to really know.
One thing though, Orton will probably need to step his kissing game up a notch. I hear it sucks. Not that I really care, but if you're going to be romantically involved with her it better make for good tv. Aggressive, I don't mind since Raw is supposed to be raw, but please not that sloppy stuff Edge and Vickie had going on.
rant
I'd really like to see where they can take this and yet I feel as though the whole rating ordeal may limit the possibilities of this potentially amazing storyline/era. Let Smackdown be PG. Come Monday night at 9, put the children to bed and let Raw be raw. They have school to go to on Tuesday. If they want to stay up and watch, it won't be because Rey Rey just pinned the last big man on the roster, it'll be because Rey Rey just got tossed out of the ring and into the second row and Orton is whooping the boss' ass and banging his married daughter. You know, the stuff the grown ups watch for. Hell, even Hannah Montana hangs out with the 20 year olds. Gear the program toward the 18-24 demographic and the 13-17 year olds will [continue to] come. Not the other way around, aside from the die-hards and the ones who are glued hoping to see the transition. They're already capitalizing on new age media. Use that together with the things that made your superstars household names and grabbed mainstream attention in the late 90's.
/rant
Anyway, we'll see how it goes.