currently, and from what we have seen from the dev diaries for RTP this is still the case, all landless titles are at duchy tier. It would be interesting to be able to have titles without lands at kingdom tier, empire tier, etc. because this would allow them to be inserted into a hierarchy (de jure or de facto), which would be extremely useful for modding.
#Landless title tier hierarchy
indigo holly
simple ruin
Emperor would be a bit odd, but I think you could find a modding use