Second machine help

Hello all, long time lurker, first time poster here. With the help of so many contributers in these forums I have successfully built my first cnc router. It's not pretty but it cuts well and I've had a blast doing it.

I am trying to decide where to go next. My current router has a cutting area of about 15"x15". I would like to use it to build a larger router but I am not sure which design I should be looking at since I am limited in the size of the parts I can cut. Any advice would be greatly appreciated.

Thanks in advance.