I "was" going to make a new version of Siege, I would call it War Siege or something similar, where the objective would be extend the gametype to a point where it would indeed support online vehicles, justify bigger nukes, air strikes and base weapon systems, as entire buildings and towers could be built and all.
Then I took a good look to the amount of active Siege players and new ones joining in a daily basis, and how most of them react to new Siege versions, then I lost my motivation to do anything at all.
So in the end, I would rather do a whole new game with a similar concept in UDK rather than do anything Siege related for UT, the amount of active players and the typical bad attitude towards everything a developer makes grinds my gears.
Sam is the leader of uK, and he can confirm this to you (he knew some of my plans for Siege, and some of the reasons why I didn't go forward with it).
NW3 Final will be my last mod release, sorry.