World Heavyweight Championship: Chris Jericho (or) Kane (c) v. the Undertaker:
I can only hope this match happens for Smackdown. Jeff Hardy likely won't hold the Championship behind Summerslam, and forbid if he manages to hold it to the Hell in a Cell show.
I suppose the world of Hardy marks will blow a nut over the thought of Taker/Hardy - inside the Cell, but the truth is, I see the Undertaker taking on a big enough talent for the Championship that it'll Main Event the entire show. And the only two individuals worthy enough to become Champion, and face off against Taker, would be Jericho, or Kane. (Kane - just because of his history w/ Taker, and his recent domination)
W.W.E Championship: Randy Orton (c) v. Shawn Michaels v. Triple H v. John Cena:
I doubt amazingly this type of match will happen. Its more likely to be Legacy against D-X, in a Handicap clusterfuck, with the title being on the line if anyone can pin Orton. But to be honest, I'd rather see a decent/great Fatal Four Way, than a possible Rhodes/DiBiase involved Main Event match-up.
Now then.. these are the two matches that are possible, and very likely. Of course the Undertaker should be a lock for the Cell, as-is an Orton/Triple H style match of some kind.
So, the odd-ball match-ups that I'd like to see, since its an "All-Cell" show.. would be as follows.
Diva's Championship: Maryse (c) v. Mickie James v. Beth Phoenix:
How amazing would Beth be inside a structure designed for torture? I think she'd re-find the amazon inside her, or glamazon, whatever.. and tear apart the W.W.E's most beautiful Diva, and it's most seasoned vet at the same time.
Women's Championship: Michelle McCool (c) v. Natalya:
See above regarding Diva's title; replace Beth w/ Natalya..
Intercontinental Championship: Rey Mysterio (c) v. John Morrison v. C.M Punk:
Punk and Morrison going against each other with each one having more experience and gained more support should be amazing. Adding a seasoned vet like Mysterio into the mix would be a recipe for excitement.