So i though about making a topic about "themes" in the first four class articles, but then i remembered that i already made this topic.
First of all i don't think that subclasses are entirely built around needs for certain themes, as some people are predicting. At least not in the sense of every class getting something based on fey or death. Giving something fey related to bards and druids made sense, as spirit related stuff for barbarians and druids. Rather i think they are Cleric is perhaps the best example of new subclasses that were class expansions into concepts that the core domains didn't cover. Same with barbarian. The only one that seems to be popping up more than others is the shadowdeath theme, but its less about shadow and death and more about the need to give some classes access to some darker or anti-hero types (cleric is a sort of inversion with the grave domain actually).
Still there are themes that might be visited again under some classes, but where they will make some sense.
Death: I'm looking at this and shadow theme separately, because i dont think they will be always necessarily connected (although they can be). Not sure what else we might see under it. Most classes that should have it, already have. Sorcerer? More anti-undead Ranger?
Divine: Maybe some class gets a divine related option, not sure which. I don't think that it will be a 1/3 divine casting option for fighters.
Elemental: It can be argued that the storm barbarian is an elemental option. Other than that we haven't seen any other unless you consider forge domain one (and it can be certainly used as a fire or metal domain). I think we might see an additional elemental option for sorcerers (similar to stormborn) and warlocks (hopefully sha'ir or genie pact). An elementalist or wu jen option for wizards could be nice, but i'm feeling doubtful. I could also see an optional or revisited version of the way of four elements for the monks.
Fey: Ranger and Sorcerer are the only classes i see getting fey related subclasses. Others mot so much. Maybe a fey scholar or witch subclass for wizards.
Shadow: A "way of shadows" option for rogue, maybe a new version of the shadow sorcerer, maybe something for warlocks or wizards .
Spirit: I think we have seen all of the spirit theme. Maybe we get a option for monks, perhaps a "Tatooed monk" who channels animal spirits. Nature connection and primal theses give some possibility to ranger, but i feel doubtful.