I like your idea of one player become infected and the rest stay normal...And I also wonder if the dlc they will have out would be free or you have to buy it,Since I got some exclusive stuff today at gamestop I wonder
Normally I don't care about multiplayer, and in a story-based single-player game like TLOU, would normally conside it "slapped on" in the same way as it was for Spec Ops: The Line.
On the other hand, there are pretty much zero post apocalypse MP games, and I do find the idea interesting. If handled right, and done differently to the same old "PvP running around scoring frags with a capture the flag (can imagine players slinging poor Ellie over their shoulder and running her back to their base!) modes thrown in"... it might actually be something rather fun.
IMO, the "obvious" way to go would be humans vs infected, with a human vs human "1-on-1 deathmatch" mode too. Yawn.
What I'd like to see really is something more post-apocalpse themed, a fight over food supplies or something, rival bandits trying to out-loot their fellow survivors and/or either teaming up or choosing to kill each other (dynamically, rather than "teams"). Letting players communicate and form ad-hoc alliances where one might suddenly shoot the other in the back once they find a gun, or could work together to take down everyone else and share the supplies, would be an interesting form of PvP, I think.
Throw in some AI controlled infected optionally too, if needed, on some "maps", but keep the areas large and open so players can go awhile before stumbling across each other, and give incentives (no respawns?) to hide, back off or surrended items so that players are actually careful about engaging each other and either need to make allies or find decent weapons/ammo to defend themselves.
For added depth, add a kind of levelling that lets survivors of a "match" keep some/all their found gear, which they can modify as in SP (but perhaps lose if killed, as part of encouraging caution?), and even form "gangs" between friends.
Just idle thoughts, obviously, on the kind of MP I actually think would be different given the setting of TLOU.
Add supply raid to the game modes, in the trophy list, one of the trophies says play one match of supply raid, so I'm assuming that's going to be a MP mode.