Just for starters and for the record I do not endorse the idea of a 2nd Rey Mysterio title reign. Not now, not ever. I believe the obvious objections have been covered by all of you quite well so I won't bore you with schematics and a regurgitation of the things you said on the matter.
Rey had his place in the sun for a minute over other guys who could have carried the title better than he did. His matches as champion were not convincing in the slightest, in turn devaluing that belt and the mantle of champion in general.
Against guys his own size and slightly bigger he is very spectacular, you can't take away what the guy definitely has got. He is a great high flyer and a good all around performer. I just don't think his place is amongst the big men. Go ahead and call him the biggest little man in wrestling history, he probably is, but not World Heavyweight Champion.
The only instance in which I see that would be appropriate for a Rey Mysterio title reign would be if it was a transitional title change. You guys all mentioned the superstars who he could have great matches with as champion now. Morrison, Ziggler, Punk, Jericho, Swagger??? That would be the only useful facilitation of a Rey Mysterio championship. Having him hold the title and go through those guys would give them a rub with the title and possibly put them in the big leagues, but I think you can still do the same thing without it by simply giving the title to any of them at this point.
There is a decent size difference between Mysterio, Ziggler, Morrison, Jericho, Kofi, Miz, Punk, other guys like that. And then there's Triple H, Undertaker, Batista, Big Show, Khali, Kane, so on and so on. Just mixing up the title with some of those smaller heavyweights kind of puts them all in a new class, an upper class. It's kind of like that period of time with Bret and Shawn as champion. They aren't the biggest guys, but they are big enough that you can believe it when they stand toe to toe with one of the real big men. Cena is perfect because he is small enough to fight the smaller faster guys but powerful enough to fight the biggest guys, kind of like Bret and Shawn and another person I'd like to mention in there Mr. Perfect.
Using Rey to put one of those guys over would work. Rey would have another title reign added to his total. And the doors would be open to more competitors for the title fixing some of the problems mentioned today about the same people always being in the title picture. I would have it be a quick title reign though like I said. The real trick at this point would be getting the title on him. You can't take the title from Undertaker to give it to Mysterio, that feud doesn't need to happen. C.M. Punk would have to win it from Undertaker somehow or possibly Jericho.
Ah Ha! Here you go. Undertaker and C.M. Punk could feud through Bragging Rights, Survivor Series, and TLC: Tables, Ladders & Chairs , into Royal Rumble. That might seem like too long but that's only a 3 pay per view title reign. If you wanted you could even have another title change and give C.M. Punk the title back at Bragging Rights, Survivor Series, or TLC then have the rubber match at Royal Rumble. They key is having it be one of the last matches in a series between Undertaker and C.M. Punk with Punk winning it to get it off of Undertaker so we can get it to Mysterio eventually in a credible manner.
SO, moving on. Let's say that title match is after the actual Royal Rumble. You then have Rey Mysterio win the Rumble making him the challenger for the title at Wrestlemania. A good idea would be to have Rey eliminate Jericho from the Royal Rumble. Jericho leaves in a fit and goes on to interfere in the match between Undertaker and C.M. Punk helping Punk win the title.
Later on Smack Down you could then have him on claiming that he should have won the title shot at Wrestlemania, that it was a fluke for Rey to eliminate him. Not only that, he has a plan. Jericho challenges Mysterio for his spot at Wrestlemania in a match at Now Way Out, explaining that he helped C.M. Punk win so that after he beat Mysterio, he would beat Punk at Wrestlemania instead of the Undertaker. Teddy Long would then come out and call C.M. Punk to the ring. He would then announce that since there was interference in his win, he would face the Undertaker one last time in a steel cage match at No Way Out.
Punk would protest saying he won so he shouldn't have to fight Undertaker again but then Chris Jericho would come out. He would then remind C.M. Punk that he only won the title because of his interference and so Punk owes him. Out of nowhere the lights go out, and everyone hears the gong. The lights raise and Undertaker in in the ring with C.M. Punk, Teddy Long, and Chris Jericho. Long and Punk jump out of the ring as Undertaker grabs Jericho picking him up and choke slamming him, then going for a tombstone, when out of nowhere big show would come out attacking Undertaker and saving Jericho from the tombstone.
The next week you could have C.M. Punk come out and start talking shit about Rey and his odds of beating Jericho or him if he even makes it to Wrestlemania. Then Rey would come out, play the face and talk about how he's going to win, and then out of nowhere have Jericho intervene. He could come to the ring and officially challenge Rey to fight him at No Way Out for his title shot. Rey would then accept, telling Punk he is next after Jericho if he can make it past the Undertaker. That night Teddy Long could make a triple threat match between the three of them for the main event, non title of course. Rey could win that match to give him a little push into next week. Again Undertaker appears attacking C.M. Punk this time.
For week three leading up to No Way out on Smack Down you could have Jericho and Punk talking to each other, they agree that Rey and Undertaker are a problem for both of them because Jericho wants the title shot that should be his at Wrestlemania, and C.M. Punk doesn't want to loose the title. They could ask Teddy Long to make a match, to which he agrees to have a 2 on 1 match for the main event. C.M. Punk and Chris Jericho v.s. Rey Mysterio. Mysterio loses being overwhelmed by Punk and Jericho. After the match Jericho and Punk celebrate when once again the Undertaker appears and this time takes out Punk first and then attacks Jericho, only once again the Big Show comes to the aid of Jericho beating up Undertaker and attacking Rey Mysterio. Big Show and Jericho start beating up Rey and then Batista comes out to save him beating up Big Show as Jericho runs away. And this takes us into one week before No Way Out.
On this show, Batista would come out say Big Show is just a bully who likes to attack people from behind and use numbers with Jericho's help. Batista then challenges him to a one on one match with someone who is big enough to beat him, not Rey Mysterio after taking a beating. Show comes out, accepts and choke slams Batista. Teddy Long would then decide to make a six man tag match with Undertaker, Batista, and Rey v.s. C.M. Punk, Chris Jericho, and Big Show. The match concludes with a big brawl in the ring and the referee stopping it as a no contest.
Finally at No Way Out Jericho and Mysterio face off with Jericho winning with a Codebreaker and earning his shot at Wrestlemania. Big Show would beat Batista with by hitting him with 3 choke slams in a row to get the pin, then during The Undertaker v.s. C.M. Punk match he would distract Undertaker from outside the cage, allowing C.M. Punk to hit a GTS and climb out of the cage.
Now, you could have Big Show challenge Undertaker for the streak, Jericho v.s. C.M. Punk for the title, and then put Rey in the Money in the Bank match for Wrestlemania. Big Show looses to Taker to make the streak 19-0, Rey wins MITB, and after a long brutal fight Jericho beats Punk for the title, only to have Mysterio come and cash in the MITB and defeat Jericho at Wrestlemania.
That's about the best way I could think of to do it. After that, you could do whatever you wanted but I would still make it brief and get the title on to someone else within a few months.