Additionally, not only do some of the headcases retain memories of the 'host', they sometimes retain skills as well, and there is at least anecdotal evidence that in Awakened/Emerged types, they may be able to tap into the host's abilities.
Honestly, at this point we really don't know enough to know what is or isn't possible. All we do know is that it seems nobody is safe. Even dragons might be at risk. Yes, think about what a dragon headcase could do. You're welcome for the nightmare fuel.
At this point, the only ones who might be safe are materialized or possessing spirits, as well as digital entities (AIs, free sprites). We don't even know what happens if bugs are infected with CFD, or whether they can be infected at all. So yeah, as far as we know, anything with a physical body could be fair game.