I only really find the WWII German U-Boats interesting. For all the others, submarines were peripheral or no great challenge. I certainly am not interested in the Pacific Theatre.
And if an SH3 expansion promised all that was suggested in the original post, I would pay a full 50 euro for it, without question.
I vote for SHIII expansion pack.
I think the ability to hunt with other subs in wolf packs would be a big plus for nearly all of us.
Plus getting different types of messages from BdU would be nice instead of only getting "Be more aggressive" & "Keep up the good work".
Would be cool if they could somehow make the mission builder a little more "user friendly". There would be alot more people in multiplay if co-op missions were easier to create.