I think if we make a tremulous with Xreal engine, it will be the same game but with new graphic.
But if we make a trem with UDK, we should try to make something different.
For example: vehicles, opened area, new aliens, and so much more that we are able to do with Unreal Engine 3.
Currently I'm writing something like a big list of all the stuff to a game project I called " Tremulous Reload" for U3.
Maybe it will be done, maybe not, but I'm sure the idea I get is not bad.
Finally I will give you the stuff list when it will be complete.