Over the course of the series, ben not only fights new aliens but get some new aliens as well. However, he has not used any of his ultimate aliens throughout the series so far.
He will have to face some old and new villains. Kevin and Gwen have left bellwood and Max Tenneson has gotten a partner for Ben to replace them. His new alien partner is [Rook] and he is armed with a weapon called a proto-tool, his alien race has not been mentioned yet in the series.
There are many new ( and old for that matter ) aliens introduced throughout the series. The following list is of all the new aliens ( not all the aliens ) that Ben gains throughout the series: Crashhopper, an alien resembling a giant grasshopper who has incredibly strong legs which enable him to launch himself great distances or deliver very powerful kicks, Gravattack, an alien with gravity powers able to make himself and others weigh tons or weigh ounces and even has his own orbit capturing anything in it, Bloxx, an alien made of what seems to be LEGO bricks and is able to rebuild himself and also to mould himself into different things and even simple machines. There are more but I cannot remember them all so please edit.