I get what your saying, it would basically be hard to top the first one in this series (If TLoU becomes a serieis) and it would be stupid for Joel and Ellie to leave Tommys camp but who says Tommys camp is still their? Who says Tommy didnt get bitten on the way home and infected his whole camp (boom sequel opener in a matter of seconds from an unimaginitive idiot). Thats the beauty of sequels ESPECIALLY in this games world. Anything could happen anything.
Btw Joel dying and Ellie growing up err I dunno the best part of Ellies appeal is her innocence of being a child / teenager if she grows up she will just become a reskinned Tess.