Just going to make this short, because this thing would be insanely broken. There are so many powerful threats who would be even stronger if they received a Quiver Dance, coughNidoqueencough, not to mention that Venomoth can do pretty well on its own with Sleep Powder and Tinted Lens. If anything is going to be retested, I doubt it will be Venomoth.
Before I played with it in RU and saw it in UU a few times, I really wanted Cresselia to come back. It is exactly what stall and defensive balance teams need to become good again. Cresselia come in on a plethora of physical threats due to its resistance to Fighting and immunity to Ground-type moves and cripple them with Thunder Wave or set up Calm Minds. It also has access to dual screens, which can make it even harder for these threats to KO. How Cresselia really helps defensive teams is that it is an amazing Nidoqueen counter—252/0 Cresselia isn't even 2HKOed by Nidoqueen. Cresselia can also counter Moltres with a bit of investment and cripple it with Thunder Wave or break its Substitutes with Psychic. Cresselia is also an answer to many, many more threats in RU, and there's no doubt that stall would be so much better with Cresselia.
However, from what I've seen, Cresselia is sooo hard to take down, maybe even too hard. For example, people have been mentioning Spiritomb as a counter, but Choice Band Spiritomb only does 49.09 - 58.1% to 252/252+ Cresselia with Sucker Punch / Pursuit (when fleeing). Cresselia actually beats Spiritomb if it carries Toxic! DRAPION doesn't even 2HKO 252/252+ Cresselia with a Life Orb-boosted Crunch. Most of the things that people are saying that beat Cresselia, like Spiritomb and Drapion, can be worn down very easily, and Cresselia can switch out without much of a problem as well, only losing 50% from CB Spiritomb's Pursuit which it can recover with Moonlight. Besides Scyther, I can't think of much else that can beat Cresselia without boosts except for Escavalier I didn't look that hard though /lazy. Escavalier is probably the best counter to Cresselia, since it has a 68.75% chance to OHKO Cress with Megahorn. During the mini-tournament, however, I used Hidden Power Fire Cresselia, which was a guaranteed 2HKO on Escavalier after Stealth Rock. It's quite easy to predict the Escavalier switch-in as well, so I wouldn't say that Escavalier is as reliable as everyone else is saying.
I realized that Cresselia may be too bulky for RU when facing it in UU. My Scarf Heracross's Megahorn only did 60% (the actually damage is 54.5 - 64.86%) and Cresselia KOed it, as it was weakened. I didn't have much trouble taking it down after that, but I was thinking that if Heracross only did that much, then the weaker RU Pokemon would be doing much worse. Cresselia has much more switch-in opportunities in RU than it does in UU, and I think that it would be a little too overpowered.
I mainly discussed the defensive set, but Cresselia is really versatile and can run other good sets too. Dual Screens and Sub CM Cresselia could be extremely annoying as well in my opinion, and I believe that these were some of the arguments brought up in the round where Cresselia was banned. It's easy to just say that Cresselia will bring balance to RU based on speculation, but I think that if we play with it, viewpoints will be much different.
I would like to bring Durant back to be honest. My old teams were almost all defensive or balanced, and things such as Steelix and Poliwrath had absolutely no problem dealing with Durant. However, now that it gained Superpower from BW2, I think that Durant could be another one of those Pokemon that trample defensive teams. I do realize that RU is primarily offensive, though, and that's why I think it deserves a chance. Durant is extremely frail, and with all of these powerful Pokemon running around nowadays, I think that it will have trouble switching in. Pokemon like Sceptile, Accelgor, Tauros, Swellow, and Archeops are all somewhat common on offensive teams and they can outspeed Durant. There is also quite a bit of priority, and even though Durant resists ExtremeSpeed and Sucker Punch, they can still revenge kill Durant if it's at around 25%. I'm really not sure about Durant but I'd at least like to test it. Hopefully the current mini-tournament will help me take a more defined position on it!