According to this document:
http://amp.rokket.biz/docs/snorting_bull_decals_4.pdf
Prien got his emblem the Snorting bull after:
1. a major success in infiltrating Scapa Flow
2. a joke about bull fighting
and
3. the emblem reminded about a comic strip figure the crew liked
4. the emblem reminded about the character of the commander
I don't have an idea for a good emblem, but I do think a good emblem would tell a story about the boat's successes, be positive and remind of the humorous side of things, would be liked by the crew and would be a metaphor of the boat's commander. So, developing a new emblem would envolve some role play elements as the game has no interaction between the crew and the player.