Ben 10: Multi Trixes (BTMT)
Ben 10: Multi Trixes is the first series in the franchise that picks up a year after the ending of Ben 10: Alien Force. Ultimate Alien is not canon to it, although some UA events happened differently outside the series. Its fourth season takes place a whole year after the rest of it.
It is in the middle of its fourth season, and a first-season-only reboot is in progress.
Ben 10,000: Multi Trixes
It will get a sequel after its end called Ben 10,000: Multi Trixes that follows Ben 10,000 with the All-Power-Trix as he fights new villains. It will be extremely short, with only two seasons.
Click here for its page on Ben 10 Fan Fiction.
Rumored Third Series
It is rumored to get another sequel, but nothing is known about it.