Yeah, I think "Splicers" as an enemy group has barely been touched. When you are talking about genetic manipulation, the field is wide open.
I'm hopeful that Fontaine Futuristics is a big hint about what we will see. Why? Because they did plasmid research. So the possibility of unique, extreme Splicers is very high. As are test animals. Also, the chance for new automated security is also high. Remember that the rest of Rapture's weapons were ramshackle because the war took them by surprise. Fontaine was gearing up for it and would surely have a dedicated weapons program (maybe covert) in his labs. Both for defense of his secrets and for possible later sale.
I've said as well that I'm totally jazzed by the idea of sea floor levels. I want to see what wild creatures, who have been dining on Adam-rich sea slugs their whole lives, would look like....
EDIT: I was thinking about sea floor encounters, and a great image popped into my head. You are walking along and find a BD helmet. Nearby, you see maybe an arm or a leg. As you examine them, you notice that more and more parts appear...until you find a cave who's entrance is strewn with bits of BD's. Heh, heh. Something has been snacking on your brothers. And normally you'd pass on by, but you are sure that the pile of carcasses must contain precious ammo and maybe even a new class of weapon. So in you go....
