Alex Scarrow has announced that he is working on a tenth Timeriders novel. Nothing much is know about it at this point.
When talking about future projects on his website he said,
"TimeRiders10 (and more):
Yes....I am planning that. I intend to write that later this year. At the moment, no publisher has been contacted regarding publishing that. But that process will begin when, and only when, I've completed the book »
At a recent book-signing Alex Scarrow confirmed that TimeRiders10 will pick up where The Infinity Cage left off and the "four main characters" will reunite. He also revealed that TR10 will be the the first book of a second arc/series with more books to follow.