I entered and am working on my team (breeding, EV training, etc) now. I doubt I'll win very much, but I'm hoping for that special pikachu at the end of it all (mostly cause the other prizes are terrible; a Light Ball? You can get that in game easily. PokeMiles? Pfft...just participate in one Wonder Trade Wednesday or use PokeBank and you'll get more than the prize gives).
But, yea...agreed that Rotom is going to be insanely popular. Rotom-W is already popular in doubles from what I've heard, and the other one which will prolly be common is Heat Rotom. I'm considering whether or not to use one of the rotoms; they are fairly bulky, but their bulk is a little overrated since they sit at a base 50 stat for HP. Still, they can take hits pretty well, and there isn't much which can OHKO a Rotom-W in the Pikachu Cup since Grass Knot will do next to nothing to it...
...and if anyone uses HP Grass, I'm going to presume they hacked their mon because it's one of the harder HPs to get (along with HP Ground).
Easiest ones to get are Dark and Dragon; I've also gotten HP Ice on occasion, but the others are extremely more difficult to legit breed.
I bet Stunfisk will show up in team previews, at least, since it's one of the two electric types which gets a powerful ground-type attack. That thing is a beast in terms of bulk. I've used it on Showdown in the UU tier, and it wasn't OHKOed (or even 2HKOed) by a STAB scald and managed to take out a couple mons.
Unfortunately, it does have a lot of weaknesses (grass, water, ice, ground); people are going to be carefully building their teams to be able to handle the big threats such as Rotom (Wash and Heat), so it wouldn't be unusual for someone to have coverage for Stunfisk (heck, Rotom-W checks and counters it). It's also worn down very easily by status; I'm gonna throw a toxic or burn on any I see.
The time limit might be a slight issue. 60 seconds to pick your team, and 30 to pick your move; that alone nearly made me not enter.
Edit: Anyone who wants to test their team, Showdown has a dropdown option for the Pikachu Cup. Just beware of two things:
1. People WILL be packing HP Ground...cause it's Showdown
2. Unless you turn the timer on or self-time, there's no way to make sure you've made your decisions in the allotted time.
Frankly, if I wind up against someone in the actual competition who runs HP Ground on anything, I'll figure that there's a 99% chance they hacked their mons (again, that's a hard HP to get through legit methods).