Loki and Prometheus don’t read, to me at least, as even the same archetypal characters. In Ovid’s Metamorphoses there is the suggestion that Prometheus himself may have created man (“by mixing new-made earth with fresh rainwater”) which makes him a patriarchal and nurturing character. I am not familiar with the Norse myth where Loki steals fire (it has been a while since I took a course on mythology) but he does not seem to fit the role of caregiver. Both are clever characters, surely, but Loki is a trickster and Prometheus is more of the scholarly sort bent to share his wisdom.
There is an interesting parallel where both characters are scheduled for eternal punishment (Prometheus for stealing fire was sentenced to have his liver eaten by an eagle while Loki was sentenced to have poison dripped on his face for murdering Baldr and refusing to cry to ensure the fallen god’s resurrection.) Neither Prometheus or Loki are exactly gods (they are a Titan and Giant, respectively.)
But the comparisons run out… Prometheus is rescued by Hercules and I forget where he goes after that, really. Loki, however, brings about Ragnarok. Not exactly the hero of man.
It’s not uncommon for mythological figures to share aspects of their stories. Baldr, Bacchus, and Christ all have resurrection in common but I would not consider them the same character by any stretch of the imagination.
It’s fun to ponder, though.
EDIT: I neglected to take into consideration that I am focusing far too much on the Roman myths and not the Greek ones. Prometheus is definitely more of a trickster in Hesiod (deceiving Zeus with the bones and THEN stealing the fire for men) but I still think it is too big a leap to consider him the same character as Loki.