I imagine a new rapid spin would probably be of one of the less offensive types, like flying.
Now to talk about something interesting, what do want to see in terms of WiFi function? I'm hoping to see a WiFi underground. The game throws together a number of random people who may then partake in underground activities. Also, a random battle function like the one on battle revolution would be nice. As long as they give the option for an Uber ban function like in battle tower.
I suppose they could go with Electric, Fighting, Ghost, Ground, Poison, or Psychic for a Rapid Spin type if they still wanted a type to be immune it. Fightning and Poison wouldn't be good though since Ghosts still block Fighting and Steel types are extremely common. No matter what type they go with, it will still give anything a free switch-in unless it's 4x weak to it or they give it more Base Power.
Another way to nerf Stealth Rock is to give a Pokemon a Rock-type Volt Absorb ability. That way, not only does it remove the rocks for free, it also gains back health and can actually make it benificial to have your opponent set SR up. It would be better than a different typed Rapid Spin, IMO, because you wouldn't have to waste an attack removing something that only took one turn to set up. It wouldn't be able to remove (Toxic) Spikes but those can't be thrown onto a team as easily and have the same effect.
I like the idea of random Wi-Fi battles. It might actually get me to play online instead of just going on Shoddy. Garchomp and Latios would probably run rampant if there were a Battle Tower like ban and if there's no Sleep Clause, Breloom might be hard to counter as well.
The only reason to reduce a Pokemon's level would be to EV train Lv. 100 Pokemon and I'm pretty sure there's already a trick to let you do that. I suppose you could do that on the Lv. 100 Magikarp in the Resort Area to get a Lv. 100 Gyarados, but you'd need Heart Scales to teach it any of its good level up moves. You can already use Heart Scales to get Deoxys any move it wants from any of the forms so that's not really a problem. You could conceivably use it to get something like a Lv. 5 Staryu with Hydro Pump for Little Cup or a Lv. 1 Mamoswine for Endeavor but I'm pretty sure the Pomeg Glitch already lets you do that (or at least a Lv. 6 Mamoswine anyway).
This is probably just a coincidence, but Murkrow and Misdreavous were version exclusive Pokemon in Fire Red and Leaf Green and got evolutions in DP. Baltoy/Claydol and Gulpin/Swalot are version exclusives in HGSS so there's a chance we'll see Claydol and Swalot evolutions in Gen 5.