Sheep Wreck Island and Tower of Time both come with an expansion piece that gives you a new level. (and they come with 2 magic items and a new character each).
Other than that, you'll need a character of each element. Right now, you have Fire (Blast Zone), Water (Wash Buckler) and Life (Ninja Stealth Elf). There's 5 more, Undead, Magic, Tech, Air, and Earth. To save you the trouble, you may as well try to hit the other elements by getting the other Swappers where you can, who you'll also need to access all areas. I'd say your best bet is to get:
Rattle Shake (Undead, Bounce)
Hoot Loop (Magic, Teleport)
Magna Charge (Tech, Speed)
Free Ranger (Air, Spin)
Rubble Rouser (Earth, Dig)
Stink Bomb (Life, Sneak)
Blast Zone has the Fly ability and Washy has Climb so if you got the guys I listed, you'd be set, although you can get whatever you want. To kind of quickly tl;dr it:
There's gates in the game that are locked away requiring one of three things: Either an Swap ability, a single element, or a dual element. Dual Element gates can be activated if you have a swapper with both elements, or if you have a second player. For Wii U players, this is easy; just turn on a wii mote and put another character on the portal, and get close to the gate to open it. after that you can turn off the wiimote and go back to playing. Swap Zones require a particular abilty, which you can see hovering above the area. Single element gates are easy enough, just have that element and go through.
Rubble and Stink are both brand new and may not be as common, but stores are starting to restock. I saw a bunch at my TRU and I ended up nabbing a Stink Bomb (they had like 20 of each of the new guys there!)
Target actually has a double pack with their own exclusive Nitro Magna Charge (same character, just a different color) that's paired with either Free Ranger or Rattle Shake. That could shave off a few bucks for you if you plan to go out and purchase this stuff. It definitely adds up, but if you play your cards right and buy during sales you can save a lot. From the stuff that I've gotten so far on sales and what not, I've actually saved almost $100 compared to if I bought the stuff regularly.
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My Quick Draw Rattle Shake came this week. He's pretty awesome.
The final upgrade is the same as normal Rattle Shake, so it turns him that dumb purple color.
Also, TRU members, if you spent $100 at TRU since launch and going till Dec. 31st, you should have got your code to get Metallic Gilly.
I REALLY hope TRU doesn't screw everyone over and mail it out in a bubble envelope. I know they probably will too.......