I recently purchased a Honda EU7000is portable generator and installed a 30A 120/240 generator outlet on the exterior of my house. I needed to install the 30A 2 pole CB in the top 2 right hand spaces in order to install a main/generator CB interlock. I was pretty much out of spaces and didn't want to install any additional tandem breakers, so I picked up a 200A panel w/o main breaker and fed it from a 100A breaker in the main panel.
You do need to isolate the neutrals in the sub panel.
Now I've got plenty of room for expansion.
