Warning: Complicated and long post coming up.
Its all about balance, not overkill.
If you look into the main event and the upper midcarders that could substitute for main event people if need be, the WWE has 24 people for all 3 shows. There's a balance between them even if its unintentional, for the most part, that needs to be maintained, and this is one of the reasons when rosters can seem like garbage (particularly because of a need to have ECW take some of them instead of relegating 12 for Smackdown and 12 for Raw....which would make 6 heels/6 faces for each show).
You can't have Chris Jericho and Edge on the same show, as both of them are top heels that get lots of mic time and have rather similar movesets (generic, not a difference between a high-flyer and a monster).
Undertaker and Batista are essentially equals in a lot of ways. Both can be booked as the "big man face" on the brand to face people like Big Show and Umaga, but both can also work with smaller guys like Orton and Morrison.
Big Show and Umaga is an example of two monsters being on a show and having no purpose for both, while Raw currently has no big monster heels. Big Show would be better served on Raw at the moment while Umaga is on Smackdown and Mark Henry is on ECW. All three shows get a big guy.
In a lot of ways, you can balance out Raw and Smackdown getting a combination of Hardy + Kennedy / MVP + CM Punk. All three of them are upper midcarders essentially that could easily be pushed into the main event, but if you had all 4 on a show, it'd be overkill, as that'd be 4 young and inexperienced guys of roughly the same size and age.
So this thread talks about Cena being moved to Smackdown. Well, the only way that can happen is logically for Triple H to move back to Raw, as HHH and Cena sort of balance each other out, since HHH isn't willing to be the one who takes a back seat to being top face on a brand (like HBK was on Raw, sitting behind both Cena and Batista).
Cena being moved over to Smackdown would've worked better when the brand was on the CW network as that was available to mostly everyone in the country as it was a non-cable station. HHH, as much as he wants to be, is not a mainstream name. His move over to Smackdown didn't do as much for it as Cena would've had. For the appeal of the kids as well as balancing out the wrestlers strengths and weaknesses, and ignoring who has been on what shows and what rivalries have and have not been seen recently, the ideal cards for main event faces would be:
SMACKDOWN: Cena, Undertaker, Jeff Hardy, MVP, and Mysterio
--Tell me that doesn't scream popular with the kids, mainstream appeal, the "something for everyone" appeal that they're always worried about (Cena for the average joe, Undertaker for sci-fi, Hardy for the emo crowd, MVP for the black crowd, Mysterio for the Hispanic audience). It also gives you two bonafide main event guys that could win the title at any time (Cena and Taker) with two guys that could be pushed to that spot (Hardy and MVP) and one guy that could fill in but is essentially an upper-midcarder (Mysterio). It also gives you two high-flyers (Mysterio/Hardy), one big man (Undertaker), and two generic wrestlers (Cena/MVP).
RAW: HHH, Batista, HBK, CM Punk, Mr. Kennedy
--Edgier group of people for the non-cable version. You've got your big man (Batista), your wrestlers (HHH and Kennedy), your high-flyers (HBK and Punk, to an extent). You've got your upper midcarder (Kennedy), your bonafide main event guys (HHH and Batista) with two people that could win with the slightest push (HBK and Punk...HBK thrown in to balance Hardy just because of Shawn's frequent injuries and how he doesn't want to be champion). Perfect balance from Smackdown.
ECW: Matt Hardy, Kane, Finlay
--Half the time slot, half the main event people needed. Kane is your big man, Matt is your high-flyer, Finlay is your wrestler.
WWE can't just throw Cena over to Smackdown the way it is right now or else it'll kill Raw's already suffering main event scene. They also can't trade him over in place of someone like Mr. Kennedy, because that upsets the balance. That would be like considering a Randy Orton and Shelton Benjamin trade being equal. Shelton could easily fit in the upper midcarder spot on those shows, but he's not a top heel.
Cena going over to Smackdown could make a lot of sense as its the show that isn't on cable, and if the WWE wants to go PG as much as they can, then they can use Smackdown for more of their PG things while letting more of their edgier stuff take place on Raw. But it can't happen without the scales being equal.
Fin.
