There aren’t any sequels to Transmet, but Warren Ellis is very prolific and has worked on several other comics. None of them take place in the same continuity as Transmet, but some of his series include:
Planetary - this Wildstorm Comics series has been on hiatus for over a year, but it is fantastic. There are 16 issues so far, plus a lackluster crossover with The Authority (see below). The first 12 issues are available in two TPBs: All Around the World and The Fourth Man. I have no idea when #17 will come out, though. A lot of fans are frustrated, but they’ll come back running when the new issue by Ellis and artist John Cassaday hits.
Global Frequency - a 12-issue Wildstorm miniseries, with each issue as a stand-alone “mini-action movie.” Currently in progress, with a different artist on each issue.
Stormwatch - Ellis’ run covered Volume 1 #37-50 and Volume 2 #1-11, which picked up immediately thereafter. Available in five TPBs: Force of Nature, Lightning Strikes, Change Or Die, A Finer World, and Finer Orbit.
The Authority - picked up directly where Stormwatch left off, and totally raised the stakes for superhero storytelling. Mad ideas, cosmic threats, ultraviolence, and morally ambiguous heroes who weren’t afraid to kill to get the job done. Plus the “widescreen” art by Bryan Hitch was gorgeous. Ellis wrote #1-12, which are collected in two TPBs, Relentless and Under New Management, but if you have the money, I strongly recommend the oversized hardcover, Absolute Authority, that reprints #1-12.
Finally, for Transmet, there are two collections of pin-ups by other artists, accompanied by text pieces by Spider (Ellis). One is called I Hate It Here, and I can’t think of the name of the other, but they are both required reading for Transmet fans.