Thinking about how the classes would work (particularly with the range of Flavorings of Splicers) for PvP. For 1 on 1 (or even mass fights) they have to have some kind of equivalence, or players will ALWAYS pick the loophole best class. Some Classes would obviously have uses for non-fighting (and might not be used much for PvP).
There would be the usual trade-offs : maneuverbility vs brute strength, melee vs ranged vs assisted combat(machines/traps)
Player Classes :
- Brute - strength, HP, throws stuff, physical attacks, jumping
- Houdini - maneuverbility, fire-plasmid, card tricks
- Spider/Crawler - terrain use (ceiling/holes/inside walls), thrown hook thingees, jumping
- Thuggish - melee-er (maybe gets more dodging mobility because doesnt lug around so much stuff)
- LeadHead - gun weapon using, (ones with psychically conrolled fly-bot), grenades?
- Nitro - bomb maker/thrower

Big Daddy flavors - slow move(normal), strength, HP, water-travel, armor, cant jump-up, baby-sitting, construction and repair
- physical
- grenades/rockets/guns
- proximity mines/mini-turrets
- Alpha - weapons, plasmids. HP, water-travel, armor

Little Sister - corpse gathering/blood drinking, movement thru vents, cute expressions that induce vomiting

Big Sister (NPC only ??) - corpse gather, HP, plasmids, armor, physical attack, water-travel, baby-sitting

SeaSlug (joke) -

Survivor ( ) - more techno(??)

Fly-Bot - flies, gun, limited sensor input, signals from scanners/alarms

Submarine vehicle scenarios ?

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Limitations ?
One issue is how many (and what) plasmid/tonic upgrade are allowed for the different player classes (do you want a system where everyone eventually can have all the abilities or with strong class distinctions (which usually winds up unbalanced and invalidating all the work a player put into developing)

How players might use terrain - richer terrain/maneuvering versus the floor hugging we did for the most part in the Solo games.

(Spider Splicer player chars would require more complex movement types)
Water combat ...

How much technical ability -- weapon enhancements (use as a compensation for non-plasmid using classes)
Would there be 'loadout limits' similar to BS2 MP (hauling 8 heavy weapons around is soooo realistic... players should try carrying 6 full golf bags to see what it would really be like)

The old question of : if a player builds something in the game, how is it protected when they are offline. And also dont want building to take over the game like it did (ruining) Ultima Online, where every open inch of the world map was soon covered in huge cheap houses.

In the beginning - every Splicer shall have his hole. ??? Bank ??? thats one game convention that might be a bit hard to justify in the world setting. Maybe you join a Faction and THAT allows storage space that is 'guarded'.

Innovative Weirdness would be the different classes visually perceiving the world differently (ex - like how the Lil Sister saw things in BS2), Fly-bots see outlines. (This would start getting problematic if many variations of the terrain must be created and cant be 'converted' programatically, though you can do alot by alternate shaders in the GPU.)   What kind of bizarreness would a Crawler see ??   Resources collected/salvaged should be meaningful to the game (and preferably NOT grindy)

ADAM - I suppose would be the 'money' for the game (the Rapture Dollars are more common and facilitate vending machines - though who keeps replacing inventory in them post-Lamb ?? Probably more and more rudimentary supplies.)

MMORPG with missions - how to NOT run out of them (or have them become boring/repetitive cookie cuttings) 

PvP 'Dog Eat Dog' area versus a 'safer' area where players can fight NPCs instead of simply being ganged-up-on by other players (Splicer vs Splicer, or Player vs Splicer (Splicer-play) )

Players should be ALOT harder to kill than in BS2 MP (especially NO 'one shot kills' ).   Players are supposed to be given a chance to counter the attacks upon them - some kind of tactics at least -- not just 'whoever gets the first shot in first usually wins'.

What happens when you 'die' ???   Dragged off to the 'morgue' to be reconstituted... Convenient garbage chutes located throughout Rapture... Too much Splicing and you cant really die ???  Vita-Chambers as they should have been???

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I would still aim for some kind of player modding (for terrain , maybe Splicer appearances) that would get submitted and applied to the company's servers (but then on the other hand, that puts us back to them (the company) having to build a NEW Tool Set, to have an Official  Asset Submission/Vetting Process - all things that most companies havent the foggiest Idea how to do, and even less how to do right.)