Cascada's new stuff amounts to little more than derivative, rehashings of hit songs from big name popstars, but the sound revamping is by far the wisest thing they've ever done. That euro-trashy, commercial, pop-trance sound was outdated before "Everytime We Touch" was even released. At least now they're current-ish.
It was never "unique" per say, but it was certainly more unique than the fluff on TD. (If You Can Afford Me, Self Inflicted, etc.)
Not really though. Singles that were released from OOTB were pretty much the same as the singles released from TD. With the exception of TOY, all of the singles from her debut had that "radio hit" sound.
She was island at first, but that was never her "signature sound" IMO
Her signature sound varies, because she does so many different genres. But you can always tell its her because of her unique vocals that has never left.
Black Eyed Peas
Gym Class Heroes
Fall Out Boy
Green Day
Linkin Park
Maroon 5 [come on, MLJ sounds nothing like their usual...ripoff of This Love, though I like MLJ tons]
Eminem
Nicki Minaj
and tbh most rock acts signed in the last 40 years
Predictions for the future:
Frank Ocean (you never know though; depends how his label interacts with Odd Future)