In Danmachi, at least as far as the main series is concerned, there are no longer any antagonists. Who must Bell and company defeat? Who will be the adversaries for his new friends from the Freya family? OEBD? This will serve as the final boss of the franchise but not at all as an antagonist for intermediate arcs.
In any shonen (Bleach, Naruto, Saint Seiya, etc.) there are always adversaries that the protagonists must defeat. Can you write a shonen without them? But in DanMachi, generating antagonists is a big problem, as a consequence of the level system and the need for the dungeon for this one; power is a very exclusive privilege, I would say much more exclusive than what suits the plot.
The impossibility of generating antagonists has already created some argumentative problems, for example: the reuse of ideas to provide Bell with adversaries, such as the Juggernaut Xenos, which is a bad copy of Asterius (a monster that Bell defeated and revived for the second round).
Also, the practically zero opportunity to have diverse adventures when you have a high level, for example: Leon said he wanted to go on an adventure with Bell, but what could they do? Train? Explore the lower floors or the first deep floors? Those would be mere trivialities for them, so what could they do that would be interesting...?
I was reading Astrea Record and there was one circumstance that caught my attention, namely: when Ottar explained to the war council that Evilus had a warrior who was at least level 6 and they were surprised, Finn said that it was not something so unusual, given that in the times of Zeus and Hera there were multiple families whose captains were level 6, even level 7.
This might not mean anything transcendental, but I also see a principle of opportunity there, because Omori was deliberately ambiguous regarding the final fate of these families; he could have said: "the Zeus and Hera families totally annihilated them" or "they perished against the dragon", but no, he left it up in the air what happened to them, which opens up possibilities.
Would it be a monumental plot convenience to use them in the precent? In my opinion not at all, what seems to me to be a gigantic plot convenience is that in all of Orario there are only 2 strong families, abysmally superior to the rest. Orario is the center of the world and the dungeon is available to everyone, so does it really make sense that out of all the migrants in the world, only those with the falnas of Loki or Freya can reach level 6 or higher?
The scenario that I see most coherent is that described for the generation of Zeus and Hera, where, yes, there were 2 families that stood out above the others, but they had rivals who, having the same opportunities as them, also prospered.
In conclusion, this is one of the best possibilities I see to fill the need for antagonists for the main series, do you like it? Do you not like it and would prefer other different antagonists? Are you happy with just the dungeon for everyone? Let me know what you think about it!
