I'm assuming he's coming back to have some sort of a showdown with Brock Lesnar. I doubt we see any physical contact between the two, but they could be laying the seeds down for a potential feud. I know Triple H probably shouldn't be in the ring so soon -- if at all -- after the "end of an era" match, but this seems like a VERY possible road for the WWE head down. Triple H vs Brock Lesnar is a money feud that will draw HUGE money for whatever PPV they would put it on, plus, it wouldn't hurt Triple H to lose to Lesnar, so he could very well be the first big victim to fall at Lesnar's feet.
That being said, I don't think that's what's going to happen. I'm assuming Triple H and Brock Lesnar will have a showdown, but Triple H won't be getting back into the ring just yet. Lesnar's run all over Raw, making insane demands to Laurinaitis, so Triple H may very well be coming to put him back in line. After all, it seems like a plausible scenario, since Lesnar lost yesterday, so he's really in no place to be making such demands anyway (storyline-wise). The timing of the call on the PPV might interfere with that, but it's easily justifiable.
Whatever the scenario, I'm confident Triple H will be involved with John Laurinaitis and Brock Lesnar tonight. What else would they want him to be involved with? The more star power they can add to the whole Lesnar situation the better, as Cena looks to be gone for the immediate future. Triple H can very easily shoulder the load that Cena's absence will leave for a couple of weeks. If that is the case, then tonight's (assumed) encounter won't be anything special, if not just a set-up for next week's Raw -- it only seems logical to have them follow this up. This certainly isn't a one-off appearance for Triple H.