As others have noted, Naanee will be gone. No one has mentioned Buster Davis; I suspect he'll be gone or a non-factor.
Assuming both are gone/non-factors, this year Brown stands to be behind Jackson, Floyd, Ajirotutu, and Crayton on the depth chart at WR. And behind Gates and the RBs in terms of targets. The only way I could see him having fantasy value is if both Jackson and Floyd go to other teams.
Long term I would expect him to have the potential to be a starter for San Diego, but in a #2, possession role. He does not have the speed or size San Diego prefers in its #1 WRs. While Gates will eventually decline, it is also true that Norv's offenses have traditionally not produced much fantasy value in its #2 WRs. There have been exceptions, and it's also true that Rivers is better than the other QBs Norv had previously, but I suspect Brown's long term fantasy potential is limited to a top 40ish WR.