Don't add an empty subpanel just for new future loads. You should thoughtfully move some existing circuits to the new panel, leaving room in both panels for new future circuits. In the end the sub-panel will contain all those exact circuits that you would want powered during a service outage, and both panels will have room for new breakers.
If you do it that way you can call it a "critical loads panel". It's the best way to do it.
An added benefit that any lights still on the main panel serve to notify you when power is back.
6kW is small for an ATS. The load on the subpanel can never exceed the generator capacity and that has to be managed proactively, not when an outage occurs. That is problematic if you have large critical loads that are not manually controlled. I for example have two sump pumps and two fridges, which to me are the most critical loads. It would be nuts to have a critical loads subpanel and not include those. But if they all decide to come on at once, that's 5kW even without considering all their startup draws. So if I also have some lights, the gas furnace, my router and other things on there, I'm over the limit. I can manage that with a manual switch but with an ATS I can't set it up that way on a 6kW generator.
If you're going to all the expense of an ATS and a suitable generator for it, I suggest you look at a 20kW+ generator. Then you can put almost everything in the new subpanel leaving behind only a few very large and not critical things ... central air, clothes dryer, car charger, etc.
If you prefer a smaller generator you should get a manual transfer switch (which is just an interlock for the subpanel's feed, very cheap). Then you can put more than the generator's capacity on the subpanel, and manage the load manually.