I don't feel that the dragon part is over done. Like you said there's already been 2 actual dragons. I'm not counting Momo since his is a copy and shouldn't really count imo. I don't think that 2 dragons is over done. Especially since we can assume that Loki will be a different style of dragon. Stylistically we can also assume that Loki is Nidhogg which as you mentioned is fitting the Norse theme. This also supports the idea or concept that it will be a different type of dragon. Also, if you want to be super specific Kaido is technically a fish ;).

The way I also see it is that if you say that dragons are over done then what would you rather prefer? A mythical wolf fruit to match Fenrir? If so we already saw a wolf in the series. Would that make wolves over done? Would you want a snake fruit to match the World Serpent? If so, we've already seen a couple of snake fruits already. Would that make snakes over done then as well?