Ok, to me, a good Wario Land game should be single player. They got it almost right with Wario Land Shake It, just needed a bit more craziness, I guess...
I just want to see a game with the same amount of content of Wario Land 2, the cool mechanics of Wario Land 3 and the craziness of Wario Land 4.
As for characters, bring back Capt. Syrup and/or the professor from Wario Land 4, add new ones to the series, a new villan or just use Capt. Syrup as the antagonist. I kind of liked Capt. Syrup as the rival in Wario Land Shake It, but I wish even then they had more interaction. I loved defeating her nonesense plans in Wario Land 2. Don't overdo it, just use what it worked before.
As for platform, Nintendo 3DS or WiiU are just fine, no real preference there.