This takes place two years after the first story because I assumed that's when the movie took place. Kevin, Gwen, and Ben still have their designs from Alien Force/Ultimate Alien but they are older and have different clothes. They can be Omniverse if you want reference, but if not (here are my thoughts):

Ben: Same shoes from Omniverse. The Omnitrix is also from Omniverse. Blue pants. Black tank top with a green 10. Wears a white jacket with a hoodie that he leaves unzipped with green lines that trail down the arms, lines the hoodie and zipper. And has the Omnitrix symbol on the back. Hair is a little messy but otherwise the same as Alien Force/Ultimate Alien.

Gwen: Shoes from Ultimate Alien. Black leggings with a dark-gray skirt that stops mid-thigh. A white long-sleeve blouse under a blue button up, sleeveless vest. And still has her long hair which she keeps up in a high ponytail with a navy blue hairband. Anodyte form: purple top with short sleeves and purple skirt that stops mid-thigh with ankle high black boots. Black stripe that trails down the neck-line to the bottom of her top. And cat belt-buckle and mask with purple highlights. Light purple skin and bright pink, energy that replaces her hair and eyes.

Kevin: A long sleeve, dark gray T-shirt with a black shirt-sleeved shirt over it, ripped the sleeves off. Wears ripped up dark blue jeans. Shoes from Omniverse. Hair reaches a little pass his shoulders. With a black leather jacket and some dull spikes on the shoulders.

Hope that helps but if you want to change it then go ahead. I genuinely like the sequel better and I am a little excited for this one. I hope you enjoy. Bye.