I'm hoping to see less silliness as well. If it does have team co-operation multi-player, I would expect to play as hunters/bandits surviving in one area. Adding little comical customizations such as a raptor mask or top-hat would look very out of place in the game. I think it could be a nice edition to the game if you play with friends for fun, but that's the only time I could think it would fit in. Just for laughs and such, but then again this game really doesn't look like like it would give you many good laughs and such, but more of a focused survival feel. One thing I also don't want to see is the addition of the listen mode in multiplayer, that would be so out of place using it against other players.
If it does have customization then it should have items that wouldn't make the game feel silly. Such as a worn out baseball cap, gas mask, or maybe an old backpack if you want to use a custom character.