Avatars and/or NPCs
Big Daddy — Bouncer & Rosie, as well as Delta
Splicers
Perhaps Little Sisters or Jack
Other named characters may be interesting, such as Sander Cohen or Andrew Ryan
Weapons:
This could be very simple or very involved depending on commitment to the source art style and arsenal
I included a photo listing the weapons in Bioshock 1 and 2. Infinite is basically a whole new franchise. Love it, but this isn't the place.
Golf club
It would be awesome to implement different plasmids since that's half of the series' identity.
Map
A port of some or all of the Bioshock 1 and 2 levels would be awesome! Not much to say, it's literally just... The maps from the games.
The multiplayer maps may be a good compromise for a lighter workload
Why?
I think Bioshock still has quite a fan base to delight with such a Modpack and that these mods could open up quite a bit of fan expression.
This Modpack could facilitate a boss gamemode, have a souped up Big Daddy take on waves of splicers (either AI or weak players). Also general in-character messing around, or even a genuine game port. Big Daddy's especially would be an interesting NPC since they would have such different proportions, more health, and hopefully? unique attacks (at least with that big scary drill hand)